Alumni of IMPRS-HD

The first IMPRS-HD alumni have left Heidelberg 2007. Below, please find a all PhD researchers graduated within IMPRS-HD in the years 2013, 2012, 2011, 2010, 2009, 2008, or 2007).

We hope they all enjoyed to stay with us and wish them good luck and success for their future careers!

This page is not updated anymore, please check our NEW web-page!
Alumni - 2014
Deniss Stepanovs
Launching Jets and Outflows from Magnetized Accretion Disks. Advection, Diffusion, Dynamo. (thesis pdf, 7 MB) Supervisor:
Christian Fendt (MPIA)
Thesis abstract: The results of three-dimensional, axisymmetric, magnetohydrodynamic simulations are presented, investigating the launching of jets and outflows from a magnetically diffusive accretion disk. The time evolution of the disk structure is self--consistently taken into account. In contrast to previous works, spherical coordinates for the numerical grid were applied. The new setup made it possible to run simulations for more than 150,000 dynamical times on a domain extending 1500 inner disk radii with a resolution of up to 24 cells per disk height in the inner disk. This, in fact, is the longest and resolved simulation performed so far. A novel approach to the analysis of disk-jet system was developed and interrelations between the properties of the jet and the underlying disk were derived. Using this method, it was shown that it is the actual disk magnetization that best describes the disk--jet evolution. The actual disk magnetization at the jet foot point is the main parameter that governs the properties of the jet, namely all jet steady-state MHD integrals. Depending on the disk magnetization, jet launching occurs in two different but complementary regimes - jets driven predominantly by centrifugal or magnetic forces. These regimes differ in the ejection efficiency concerning mass, energy, angular momentum, and all jet integrals. The self-generation of the magnetic field by a Alpha-Omega mean-field dynamo was also addressed. In this setup, the magnetic flux is continuously generated, diffuses outwards the disk, and fills the entire disk. A magnetic field of the inner disk is similar to open field structure, favoring magneto-centrifugal launching. The outer disk field is highly inclined and predominantly radial. The differential rotation induces a strong toroidal component that plays the key role in the outflow launching. These outflows from the outer disk are slower, denser, and less collimated. A toy--model triggering a time-dependent mean-field dynamo was invented. The general result is that the episodic ejection and large-scale jet knots can be steered by a time-dependent dynamo that regenerates the jet-launching magnetic field.
Mohamad (Moe) Abbas
RR Lyrae stars: Substructures and the Oosterhoff dichotomy in the Halo of the Milky Way (thesis pdf, 20 MB) Supervisor:
Eva Grebel (ARI)
Thesis abstract: We present the detection of 6371 RR Lyrae (RRL) stars distributed across 14000 deg2 of the sky from the combined data of the Sloan Digital Sky Survey (SDSS), the Panoramic Survey Telescope and Rapid Response System 1 3pi survey (PS1), and the repeated observations from the second photometric catalogue from the Catalina Survey (CSDR2). Around 2000 of these stars are new discoveries. The completeness level of our RRL stars (4800 RRab and 1571 RRc stars) is 50 % while the efficiency levels for RRab and RRc stars are 99 % and 87 %, respectively. We show that our method for selecting RRL stars allows us to recover halo substructures. Moreover, we present a method for selecting RRL stars and other types of variable stars in the absence of a large number of multi-epoch data and light curve analyses. Our method uses a Gaussian Mixture Bayesian Generative technique (GMM) to apply SDSS color and PS1 variability cuts instead of the commonly used rectangular cuts. A comparison between our efficiency and completeness levels using the GMM method to the efficiency and completeness levels using rectangular cuts that are commonly used yielded a significant increase in the efficiency level from 13% to 77% and an insignificant change in the completeness levels. Although we develop it over the SDSSxPS1 footprint, our technique would work well on any multi-band, multi-epoch survey for which the number of epochs is limited. Additionally, we study the different Oosterhoff populations (OoI and OoII) in the Milky Way's halo using the largest catalog of RRab stars from the Catalina Sky Survey (CSS). We show that 73% of the halo RRab stars are of OoI type (RRabOoI ) and that the latter stars are more metal-rich compared to RRabOoII stars. By examining the change of RRabOoI and RRabOoII stars with Galactocentric distances (Rgc), we detect two turning points at Rgc 25 kpc and Rgc 45 kpc. We believe that these turning points are associated with breaks in the power-laws of the halo and that they divide the stellar halo into three main components: the inner halo, the outer halo, and the ''extreme outer halo'' components. We also study the Oosterhoff populations of the RRL stars in the direction of the trailing arm of the Sagittarius dwarf spheroidal (dSph) galaxy, the Virgo substructure, and the Hercules-Aquila Cloud and we conclude that these substructures are dominated by RRabOoI stars (80%) and that the mean periods of their RRab stars fall near the boundary of the Oosterhoff gap; results that suggest that they had dSph galaxy progenitors.
John Vickers
Stellar Tracers and Galactic Exploration (thesis pdf, 8 MB) Supervisor:
Eva Grebel (ARI)
Thesis abstract: This dissertation discusses various techniques for studying the structure of the Milky Way via the use of stellar tracers. One of the most widely used stellar tracers is blue horizontal branch stars, which are prized for their predictable and substantial intrinsic brightnesses and their old ages. We begin by creating a z based photometric color cut for the selection of blue horizontal branch stars which may be used in data produced by the Panoramic Survey Telescope and Rapid Response System. Using this color cut, we select a sample of blue horizontal branch stars and pass them through a newly devised search algorithm which is capable of finding overdensities in incomplete and patchy data. From this, we rediscover a number of well known Milky Way features and discover a new constraining detection of the Sagittarius tidal streams. If we use spectroscopy in addition to photometry, we can isolate another stellar tracer, main sequence stars. A main sequence star's intrinsic magnitude is a function primarily of the mass and metallicity of the star; so with spectroscopy and photometry we can easily estimate the distance to these types of objects. We collect a sample of F-M type dwarf stars from the Sloan Digital Sky Survey spectroscopic data set. Having spectroscopy and astrometry, we may assign six-dimensional phase information to these objects and then use that information to study their bulk motions as a function of position. Here, instead of looking for large structure, we instead look to aberrations from large structure: using this relationship between motion and position as a probability density function, we select and characterize significant outliers. We also identify phenomenological objects such as hypervelocity and Hills stars. These types of stars are indirect probes of the Milky Way and are most useful for studying: the potential of the Milky Way, the locations of extreme overdensities such as spiral arms or black holes, and the structure of observationally obscure areas like the Galactic nucleus. During these studies of the motions and positions of stellar tracers in the Milky Way, accurate reports of the proper motions are of critical importance. To investigate this we closely examine proper motions in the Sloan Digital Sky Survey and the PPMXL proper motion catalogs and devise metrics for estimating the accuracy of these proper motion valuations and corrections to erroneous proper motions where applicable.
Alberto Rorai
Measuring the Small Scale Structure of the Intergalactic Medium (thesis pdf, 4 MB) Supervisor:
Joseph Hennawi (MPIA)
Thesis abstract: The small-scale structure of the intergalactic medium (IGM) is fundamental to our understanding of cosmology and structure formation. Although the baryons trace dark matter fluctuations on megaparsec scales, on small scales ($\sim 100$ kpc), gas perturbations are regulated by hydrodynamics and they are thought to be suppressed by pressure below a characteristic \emph{filtering scale} $\lambda_J$, analogous to the classic Jeans scale. The value of this Jeans filtering scale is set by the interplay between pressure support and gravity across the cosmic history, and has fundamental cosmological implications. First it provides a thermal record of heat injected by ultraviolet photons during cosmic reionization events, and thus constraints the thermal and reionization history of the universe. Second, it determines the clumpiness of the IGM and the minimum mass for gravitational collapse from the IGM, playing a pivotal role in galaxy formation and reionization. In principle, the sign of Jeans smoothing could be probed by the redshifted \mlya\ absorption lines in the spectra of high-redshift quasars (The \mlya\ forest). Unfortunately, this is extremely challenging to do because the thermal Doppler broadening of \mlya\ lines along the observing direction is highly degenerate with pressure smoothing. In this work, I explicitly show what degeneracies hold among the thermal parameters of the IGM when only line-of-sight observations are possible. For this purpose, I devised a rigorous statistical algorithm based on Gaussian processes and Markov-Chain Monte Carlo methods, trained on a grid of semianalytical models of the IGM. I then introduce a novel method able to measure the Jeans scale by estimating the transverse coherence in the spectra of close quasar pairs (transverse separation $r_{\perp}< 1$ Mpc). This method is based on the phase differences of homologous Fourier modes in the \mlya\ forests of quasar pairs, and I prove that it is maximally sensitive to $\lambda_J$ and only weakly dependent on the other considered parameters. The available sample of quasar pairs is analyzed, after careful calibration of noise, resolution, and other possible systematics. Our new method applied to this dataset provides the first measurement of the filtering scale of the intergalactic medium. A first comparison of our findings with hydrodynamical simulations suggests that the filtering scale predicted by the standard thermal models of the IGM is significantly higher than what we observe, motivating further theoretical studies to understand this discrepancy.
Jennifer Schober
On the Role of the Turbulent Dynamo in the Evolution of Cosmic Magnetic Fields (thesis pdf, 7 MB) Supervisor:
Ralf Klessen (ITA)
Thesis abstract: The aim of this work is to explore the origin of magnetic fields in the Universe. We claim that the turbulent or small-scale dynamo, which amplifies weak seed fields on short timescales in the presence of turbulence, plays an important role in the evolution of cosmic magnetic fields. The theoretical model for the turbulent dynamo is generalized for various astrophysical environments, with a focus on different turbulence spectra. We derive analytical solutions for the dynamo growth rate in the kinematic phase and discuss the subsequent non-linear evolution as well as saturation. In the history of the Universe turbulence is expected to be driven efficiently at the latest during the formation of the first stars and galaxies, where gravitational energy is converted into chaotic motions as the dark matter halos accrete gas from the environment. We model these processes semi-analytically and implement magnetic field amplification by a turbulent dynamo. Our results show that unordered magnetic fields, with strengths comparable to the ones in local galaxies, were already present in the primordial Universe. A potential observational test for magnetic fields in young galaxies is suggested to probe our proposed scenario for the evolution of cosmic magnetic fields.
Thomas Gerner
Chemical characterization of the early evolutionary phases of high-mass star-forming regions (thesis pdf, 5 MB) Supervisor:
Henrik Beuther (MPIA)
Thesis abstract: The formation of high-mass stars is a very complex process and up to date no comprehensive theory about it exists. This thesis studies the early stages of high-mass starforming regions and employs astrochemistry as a tool to probe their different physical conditions. We split the evolutionary sequence into four observationally motivated stages that are based on a classification proposed in the literature. The sequence is characterized by an increase of the temperatures and densities that strongly influences the chemistry in the different stages. We observed a sample of 59 high-mass star-forming regions that cover the whole sequence and statistically characterized the chemical compositions of the different stages. We determined average column densities of 18 different molecular species and found generally increasing abundances with stage. We fitted them for each stage with a 1D model, such that the result of the best fit to the previous stage was used as new input for the following. This is a unique approach and allowed us to infer physical properties like the temperature and density structure and yielded a typical chemical lifetime for the high-mass star-formation process of 105 years. The 18 analyzed molecular species also included four deuterated molecules whose chemistry is particularly sensitive to thermal history and thus is a promising tool to infer chemical ages. We found decreasing trends of the D/H ratios with evolutionary stage for 3 of the 4 molecular species and that the D/H ratio depends more on the fraction of warm and cold gas than on the total amount of gas. That indicates different chemical pathways for the different molecules and confirms the potential use of deuterated species as chemical age indicators. In addition, we mapped a low-mass star forming region in order to study the cosmic ray ionization rate, which is an important parameter in chemical models. While in chemical models it is commonly fixed, we found that it strongly varies with environment.
Sladjana Knezevic
An Integral View of Shocks (thesis pdf, 14 MB) Supervisor:
Glenn van de Ven (MPIA)
Thesis abstract: The subject of this thesis are Balmer-dominated shocks around (super)novae. These are characterized by strong hydrogen emission lines with a narrow (~10km/s) and a broad (~1000km/s) component. Studying shock physics is important for improving distance estimates to (super)novae, for understanding their evolution and impact on the interstellar medium, and for obtaining insights into the cosmic ray origin in the remnants of supernovae. Two historical supernova remnants, SN 1006 and SN 1572 (Tycho), are investigated using high-resolution spectrophotometric imaging. Analyzing the widths and intensities of the narrow and broad H�alpha-line components, evidence of suprathermal protons – seeds of high-energy cosmic ray protons – is found in SN 1006. Those cause an underestimation of the estimated shock velocity and, hence, the distance to the remnant. In SN 1572, our observations indicate an additional intermediate-width (~100km/s) component, implying the presence of a broad-neutral precursor. Moreover, the narrow component is typically broader than the maximally expected 20 km/s and is sometimes even doublepeaked, which might be due to a cosmic-ray precursor or the shock geometry. The last part of the thesis outlines how high-spatial resolution spectrographs can be used to study and better understand outflow mechanisms of novae, here of a classical nova FH Ser. Further development of shock models that include non-thermal physics, and their application to the data presented in this thesis, are expected to provide important constraints on hadronic cosmic ray properties.
Charles Majer
Joint reconstruction of the mass distributions of galaxy clusters from gravitational lensing and thermal gas (thesis pdf, 3 MB) Supervisor:
Matthias Bartelmann (ITA)
Thesis abstract: We focus on the reconstruction of mass distributions of the massive galaxy clusters, which are the largest gravitationally bound objects in the Universe. An approach to determine the masses of clusters is based on the effects of gravitational lensing. Estimating errors induced by this method is crucial but computationally expensive. We present a novel approach to estimate analytically the errors made by reconstructions which use weak-lensing information. As galaxy clusters host a large amount of intracluster medium they provide a multitude of observables. We present a new method to infer the lensing potential from two of these: signals of the thermal Sunyaev-Zel’dovich effect and the emission of X-rays due to thermal bremsstrahlung. By assuming that the gas is in hydrostatic equilibrium and follows a polytropic equation of state, we link these observables to the gravitational potential, which is then projected along the line-of-sight to infer the lensing potential. For this we deproject the observables by means of the Richardson-Lucy algorithm. We test our method on clusters with analytic profiles, a numerical simulation and on the galaxy cluster RXJ1347. Our efforts are the first steps towards a non-parametric algorithm for a joint cluster reconstruction. By taking all possible cluster observables into account, mass distributions of clusters will be determined more accurately.
Rahul Kannan
Hydrodynamic simulations of galaxy formation in a cosmological context (thesis pdf, 11 MB) Supervisor:
Andrea Maccio (MPIA)
Thesis abstract: The formation of galaxies and their subsequent evolution through cosmic time is governed by a variety of complex physical processes such as gas cooling, star formation, feedback and merger events. In this thesis we use hydrodynamical simulations to study the effect of these processes on galaxy properties. We first investigate the change in galaxy morphology as they undergo mergers. We look at a wide variety of merger events from interaction between small dark matter subhaloes and galactic discs, to major mergers. In the second part of the thesis we turn our attention to the role of stellar feedback in regulating star formation. A model for short range photoheating of gas by radiation from massive stars is tested by simulating a representative volume of the Universe and comparing the statistical properties of simulated galaxies with the observed ones. Finally, we introduce a new, computationally efficient model to calculate the gas cooling rate in the presence of local radiation fields. The model uses simple assumptions for absorption of ultra-violet photons and an optically thin approximation to propagate local radiation fields throughout the entire simulation volume. Using this new method we show that local radiation has a significant effect in regulating the star formation rate of L* galaxies. It reduces gas accretion onto the disc, thereby, producing realistic galaxies without resorting to extreme feedback mechanisms.
Johannes Ludwig
A survey of dwarfs and tidal debris around nearby massive galaxies – deep imaging with medium-sized telescopes (thesis pdf, 64 MB) Supervisor:
Eva Grebel (ARI)
Thesis abstract: The goal of this thesis was to search for interaction signatures among massive galaxies and to quantify the properties of their satellites. We performed deep wide-field imaging on nearby massive spiral galaxies in a distance range of 8 to 35 Mpc. The spirals cover a variety of morphologies and are located in environments of different densities and richness. We studied the surroundings of 47 massive galaxies. 29 targets were studied in detail. For all of them, we were able to detect dwarf companions. In total we found 55 mostly new candidate dwarf galaxies (CDG), on average about two CDGs per target. Based on their structural parameters and colors, we classified the majority of the CDGs as dwarf spheroidal galaxies, but also dwarf elliptical and dwarf irregular galaxies are included in the sample. The satellites’ observed surface brightness in the B-band falls in the range 22 < mu_B < 26 mag arcsec−2, the colors vary from 0.8 < (B − R)_0 < 1.5 mag, and the measured range of luminosities is −8 < M_B,0 < −17. The number of CDGs observed is quantitatively in agreement with the expected number compared to the Milky Way or M31 within our sensitivity limits. Furthermore, about 54% of our host galaxies show signs of tidal interactions. These range from prominent tidal tails, plumes and clouds to thin and faint stellar streams. 35% of the massive galaxies exhibit signs of tidal tails or bridges, 11% show plumes or clouds, and only 8% show stellar streams. We discuss possible progenitors of these features based on their colors.
Ellen Andrae
The GALEX-GAMA survey and its application to the statistical inference of the attenuation of starlight by dust in spiral galaxies (thesis pdf, 19 MB) Supervisors:
Richard Tuffs, Werner Hofmann (MPIK)
Thesis abstract: This thesis describes a new widefield survey of the UV broadband emission from a complete flux-limited sample of galaxies in the local Universe conducted with the Galaxy Evolution Explorer (GALEX). I develop techniques to reliably measure UV photometry from galaxies catalogued in the Galaxy and Mass Assembly (GAMA) spectroscopic survey, accounting for the influence of varying UV background, neighbouring sources and measurement artefacts. I also develop the framework for a statistical technique for the inference of the intrinsic UV emission of stars in the population of spiral galaxies, correcting for the effects of attenuation of UV light by dust in flux-limited samples. This technique incorporates for the first time an explicit treatment of the dependency of the attenuation on orientation, size, and morphology of the galaxies. This is a prerequisite for the use of the UV data to infer the present and past evolution of local Universe galaxies from the UV-optical photometry.
Veselina Kalinova
Mass distributions of galaxies from SAURON and CALIFA stellar kinematic maps (thesis pdf, 12 MB) Supervisors:
Glenn van de Ven (MPIA)
Thesis abstract: The amount and distribution of dark matter in galaxies defines the formation, evolution and dynamics of these systems. In this thesis we infer the internal mass distributions of 26 E-Sd galaxies using SAURON and CALIFA two-dimensional stellar kinematic maps. We compare two modeling approaches - fitting a solution of the axisymmetric Jeans equations and the commonly applied asymmetric drift correction (ADC). We show that ADC underestimates the enclosed mass by a factor of ~3-4 once random motion (velocity dispersion) becomes significant compared to ordered motion (streaming), that is, when the ratio of ordered-over-random motion is less than about 1.5. Such kinematics indicate that the stellar mass of the galaxy is not confined to the disk plane, as happens in bulges and thick disks of spiral galaxies, as well as in elliptical galaxies. Since this is frequently realized in our sample, as well as in the general galaxy population, a full line-of-sight integration as provided by solutions of the axisymmetric Jeans equation is needed to reliably infer mass distributions in galaxies. We henceforth use the more realistic Jeans models to estimate the dark matter fraction (fDM), by modeling the total (dynamical) mass-to-light ratio and comparing it with the value derived for stars (baryons) from stellar population models. We find several trends of dark matter fraction with global galaxy properties. Ellipticals (E) and late-type spiral (Scd-Sd) galaxies are the most dark matter-dominated objects, while lenticular (S0) and early-type spirals (Sa-Sb) have significantly smaller dark matter fractions. On average, fDM increases both at highesta and lowest galaxy (stellar) masses, and is smaller at intermediate masses. Further, it is also anticorrelated with the ratio of ordered-over-random motion, that is dispersion-dominated galaxies show a higher dark matter fraction. Finally, we investigate the maximum circular velocities and qualitative shape of rotation curves as a function of galaxy morphological type along the Hubble sequence. Peaked rotation curves are found in early-type galaxies, while slowly rising rotation curves are typical for late-type spirals. This confirms predictions of recent simulations of galaxy evolution with local photoionization feedback. More generally, our results for dark matter content and dynamics provide empirical constraints on current and future theoretical models of galaxy evolution across a wide range of galaxy morphological types and masses, and thereby will help us to understand the relevant physical processes.
Tobias Albertsson
Deuterium fractionation in the interstellar medium (thesis pdf, 2 MB) Supervisors:
Dimitry Semenov / Thomas Henning (MPIA)
Thesis abstract: To date more than 170 species have been identified in interstellar space, and the chemical processes that govern their formation and destruction are driven by thousands of reactions connecting hundreds of atomic and molecular species. The study of these species give us essential information on the physical and chemical processes of astrophysical environments. Studies of deuterated species give us important clues also on the chemical ages and thermal history of these environments. As we enter a new exciting era with highly sensitive measurements of the chemical cosmos provided by ALMA, there is a need for new sophisticated models to analyze the forthcoming wealth of data. The purpose of this thesis is to develop a sophisticated chemical model for studying the deuterium chemistry, benchmark it and its uncertainties and to utilize it to model increasingly more complex star-formation environments. We benchmark the deuterium chemistry model by comparing the calculated to observed D/H ratios of a variety of mono-, doubly-, and triply-deuterated species in distinct astrophysical environments. Uncertainties in abundances and D/H ratios are quantified by a sensitivity analysis, and the most problematic reactions are also identified to aid future laboratory experiments. Ortho-para chemistry has been found to have a profound effect on the pace of deuterium fractionation. The ortho-para model is developed and used successfully to study the para-fractions in diffuse clouds where we improve our understanding of the underlying chemistry of H3+ and H2. Finally, high-temperature reactions are added to the network and used in a study of the origin of Earth's ocean water and the water in primitive bodies of the Solar system (comets and asteroids).
Trifon Trifonov
Planetary dynamics and high precision optical and near-IR spectroscopy. Testing the planetary hypothesis around evolved K-giants (thesis pdf, 20 MB) Supervisors:
Andreas Quirrenbach (LSW)
Thesis abstract: The aim of my PhD dissertation is the detection and characterization of extrasolar planets around evolved intermediate stars using precise Doppler spectroscopy. I worked with a sample of G and K giants, which have been observed since 1999 at Lick Observatory. Finding planets around evolved stars, however, is an ambitious task. Giants can exhibit intrinsic long-period RV variations that can effectively mimic a planet, such as large surface spots or even non-radial g−mode pulsations. Given the fact that Hamilton spectra have relatively low resolution (R = 60 000) for line profile analysis, those phenomena cannot be excluded. My research focuses on proving the planetary hypothesis for those objects in our science sample that show well-defined RV signature consistent with one or more sub-stellar companion(s). Thus, I have applied two quantitative tests that can finally close the case for our planetary candidates: First, I have developed a data reduction pipeline for the ESO near infrared spectrograph CRIRES (R = 100 000). I have obtained precise RV from the near-IR, and I searched for consistency with the Lick optical data. Intrinsic stellar RV variations will lead to differences in RV phase and amplitude between the optical and near-IR, while, in case of a planet, both data sets are expected to be consistent. Second, I have constructed detailed χ2red grids in order to explore the dynamical properties of the fits around the best fit. Each orbital configuration from the χ2red grids have been tested with the Mercury N-body simulator. Both methods turned out to be very successful and are included as separate parts of my PhD thesis. The near-IR velocity precision is in the order of 30−40 m s−1 , enough to confirm the RV phase and amplitude, and thus, the planetary nature of most of our candidates. The dynamical fitting shows better χ2red values than the multiple Keplerian model. This is a prove that indeed interacting planets are responsible for the Doppler shift. The long term stability test shows that some systems have a stable best fit, while the rest have broad and confident long term-stable regions in orbital parameter space within 1 or 2 sigma from the best fit.
Nils Lippok
Earliest Phases of Star Formation - Physical and Chemical Properties of Prestellar Cores (thesis pdf, 5 MB) Supervisors:
Ralf Launhardt, Thomas Henning (MPIA)
Thesis abstract: With the goal of constraining the initial physical and chemical conditions of low-mass star formation, the thermal dust emission of a sample of prestellar cores has been observed with the Herschel Space Observatory. From these observations, the most accurate maps of the dust temperature and density structures in prestellar cores existing today have been derived using a ray-tracing technique. Based on this new information on the physical conditions in the prestellar cores I model the chemical evolution of the associated gas. Comparison of the models to molecular line observations reveals that CO freezes out strongly in the core centers and that even the high density tracer N2H+ is affected by depletion. I derive a chemical age of the gas in all cores on the order of 10^5 yr which is comparable to the free-fall time of the cores. Furthermore, I calculate the thermal equilibrium distributions of the prestellar cores between the two methods confirming the reliability of the ray-tracing technique. It is also shown that the agreement is good for a large range of dust models. Finally, I present ammonia observations of three prestellar cores and use them as a gas temperature probe. Comparison of gas and dust temperatures shows that both agree in the inner parts of two cores traced by ammonia while the gas is slightly warmer than the dust in the third object; maybe due to a reduced collisional coupling between gas and dust because of coagulation of the dust grains.
Yu-Yen Chang
Galaxy Shapes through Cosmic Time - Intrinsic Structure of Early-type Galaxies at High Redshift (thesis pdf, 9 MB) Supervisors:
Arjen van der Wel, Hans-Walter Rix (MPIA)
Thesis abstract: In this thesis, we measure for the first time the projected shapes of large samples of distant early-type galaxies in deep near-infrared surveys from the ground and with the Hubble Space Telescope, and reconstruct their intrinsic, three-dimensional shape distribution assuming random viewing angles. We find that the most massive early-type galaxies are roundest at all redshifts 0 < z < 2.5, indicating that at any cosmic time mergers dominate the growth of the most massive galaxies. However, early-type galaxies were on average more disk-like at earlier times, indicating that star formation in their progenitors occurred in disks, and that merging is the dominant evolutionary channel after star formation ceases. Together with the recent finding that typical early-type galaxy sizes increase dramatically between z ~ 2 and the present, the increased `roundness' suggests that classical elliptical galaxies emerge through gradual merging and accretion of satellites. Moreover, the observed increase in number density over the same time span suggests that new early-type galaxies form continuously, also at late cosmic times. Our findings suggest that these newly formed early types are generally disk-like at all redshifts, and subsequently become larger and rounder through merging. In addition, we provide a low-redshift benchmark for high-redshift galaxy studies, by deriving new stellar mass and star formation rate estimates for nearly a million galaxies drawn from Sloan Digital Sky Survey spectroscopic sample. Our novel approach combines photometry from multiple surveys that span a large range in wavelength (from 0.4 to 22 micron), and uses the latest modeling techniques to consistently incorporate the effects of extinction and emission by dust. We use these new properties to investigate the nature of the bimodality of star formation activity in the galaxy population.
Alumni - 2013
Lei Liu
A Multi-Phase Chemodynamic Galaxy Formation Model (thesis pdf, 50 MB) Supervisor:
Rainer Spurzem (ARI)
Thesis abstract: In this thesis, I present my PhD work: a multi-phase chemodynamic galaxy formation and evolution model. The model is aimed at treating the dynamics of stars, molecular (cold) clouds, and hot/warm diffuse gas individually and allowing for mass, momentum, and energy exchange between them in a self-consistent way, so as to overcome the difficulties of a single-phase description. I introduce the detailed implementation of physical processes in the model including gravity, gas dynamics, heat conduction, cooling, star formation and stellar feedback. A dwarf galaxy model is evolved for 1 Gyr. The corresponding star formation rate decreases from 1 Msol/Year to 0.1 Msol/Year. The cloud mass distribution follows a power law with a slope of -2.3. The discrepancies of chemical abundance between hot/warm and cold phase are reproduced. As an extension to the classical multi-phase model, I introduce a transition process such that hot/warm gas can collapse to cold clouds, which solves the problem of cold clouds' initial mass fraction and distribution in the multi-phase simulation. This process is proven to be more suitable for low mass systems. Also I implement an individual star formation model, in which individual stars are created analytically inside a molecular cloud with a stellar mass distribution given by a specific initial mass function (IMF). This model reproduces the life cycle of interstellar medium in a galactic scale simulation and realizes the process of star cluster formation inside one molecular cloud. The multi-phase code is parallelized with MPI and shows good scaling relations. GPUs are used to accelerate the most time consuming parts (gravity, SPH and neighbour search), which results in a speedup of one oder of magnitude for the whole program.
Dario Colombo
Gas organization in M51 - The impact of spiral arm dynamics on Giant Molecular Cloud properties (thesis pdf, 50 MB) Supervisor:
Eva Schinnerer (MPIA)
Thesis abstract: Recent observations have shown that star formation is correlated with the molecular phase of the interstellar medium. Molecular gas tends to organize itself into large and roughly self-gravitating entities called Giant Molecular Clouds (GMCs). These objects should, therefore, play a key role in controlling star formation and defining its modes. However, their physical properties, formation and evolution mechanisms are still poorly understood - especially in spiral galaxies. The new PdBI Arcsecond Whirlpool Survey (PAWS) offers, for the first time, the possibility to study the molecular gas distribution in a grand-design spiral galaxy dominated by dynamical phenomena. The aim of this thesis is to evaluate the importance of galactic environments for the gas organization. Via a thorough analysis of the gas kinematics I study the structure of M51’s gravitational potential and spiral arm streaming motions. This analysis provides several insights on the differing nature of the molecular and atomic phase of the interstellar medium. I also find evidence for a kinematic m = 3 mode that explains the asymmetry of the spiral arms. To investigate the effect of the dynamical environment on the molecular gas I have generated the largest extragalactic GMC catalog to date using an automatic algorithm that accounts for the observational biases. Differences in the cloud properties suggest that environments, and in particular dynamical effects, strongly influence the organization of the gas in spiral galaxies and provide a way to discriminate between the various mechanisms of cloud formation and evolution that have been proposed in the literature.
Maren Mohler-Fischer
Search for extrasolar planets around young stars in the presence of stellar activity (thesis pdf, 14 MB) Supervisor:
Thomas Henning (MPIA)
Thesis abstract: Planets form in circumstellar disks around newly born stars. The timescales of planet formation and migration remain poorly constrained and are a matter of ongoing debate. The disk dispersal timescale, which sets a limit to the duration of planet formation, was derived from infrared surveys to be less than 10Myr. In order to test the predicted planet formation time scales described in current theories, it is important to find planets in the age range of 1 to 100Myrs. In the same manner, non-detections can help to understand the current theories. However, the number of discoveries is still very small due to the challenging task to analyze RV signals contaminated by the high stellar activity. Within this thesis, I performed a systematic radial velocity (RV) search for planets around 21 young targets with ages between ~3 and ~400Myr and spectral types between F6 and M0. In order to account for the strong stellar activity, I analyzed the stellar spectra in terms of a variety of activity indicators, analyzing both single spectral lines and the RV cross-correlation function. For three targets, a strong radial velocity signal could be identified, which was not induced by measurement sampling. Due to the analysis of available photometric data and activity indicators, it was possible to exhibit stellar activity patterns such as cool stellar spots as origin of the signal. The simulation with a spot program allowed to reconstruct possible spot scenarios that could have caused the detected RV variability. The challenge of the conformation of a transit planet with follow-up observations in presence of stellar activity is shown in the discussion of HATS-2b, a transiting extrasolar planet orbiting a K-type star showing starspot activity. Furthermore, the discussion of a possible companion around the young eruptive star EX Lupi is part of this thesis. The detailed RV and activity analyses show the challenging task to disentangle RV signals caused by a possible companions and by stellar activity. Although no planet could be identified in the RV search around 21 young stars, it can in summary be stated that the detailed analyses in this thesis allow to put constraints on further RV surveys looking for exoplanets around young stars. This includes not only the sample size and target selection but also the observations and data analysis methods.
Stephan Henke
Modellierung der thermischen Entwicklung von Asteroiden unter Beruecksichtigung geochronologischer Daten aus Meteoriten (thesis pdf, 4 MB) Supervisor:
Hans-Peter Gail (ITA)
Thesis abstract: A numerical model is developed for the thermal evolution of asteroids and is applied to parent bodies of a meteorite class known as ordinary chondrites. This model includes sintering, as well as the influence of 60Fe as a heat source, additional to 26Al. The model is fitted to empirically determined cooling ages of H chondrites using a genetical algorithm under the assumption of an onion shell model to determine the parameters of the H chondrite parent body. Furthermore, the influence of parent body growth on the thermal evolution is examined. The results demonstrate that porous planetesimals of a few km in size are able to develop central temperatures high enough for melting. For the most part, the results of the fit to the H chondrite cooling ages show that the onion shell model excellently reproduces the thermal evolution history of all H chondrites used. The reconstructed properties of the parent body are comparable to those of models of others. According to the fitting results, 60Fe does not play a role in the thermal evolution of the H chondrite parent body. The examination of different accretion times yields that only rapid growth is compatible with the thermochronology of the H chondrites. Also, the influence of preheated building blocks is low for the case of short accretion times. As a result, the usage of the instantaneous accretion approximation in future models is justified.
Philipp Merkel
Different aspects of the interplay between light and the large-scale structure of the Universe (thesis pdf, 4 MB) Supervisor:
Bjoern-Malte Schaefer (ARI/ITA)
Thesis abstract: The main subject of this thesis is the influence of intrinsic alignments on weak lensing measurements. One possible source of intrinsic alignments are correlations in the angular momenta of neighbouring galaxies. Employing an improved ansatz for the angular momentum correlation function I show that the typical correlation length of Milky Way-sized haloes is about 1 Mpc/h which is slightly smaller than earlier work in this field suggested. Establishing the constitutive formalism to describe intrinsic alignments consistently in the framework of 3d cosmic shear I compute the resulting covariance matrices of different alignment types. For a Eucild-like survey it turns out that intrinsic alignments are more than one order of magnitude smaller than the lensing signal. In addition the parameter estimation bias in a two-dimensional non-tomographic weak lensing measurement is computed. The matter density Omega_m and the normalization of the linear matter power spectrum sigma_8 are most severely biased if intrinsic alignments are described by an angular momentum based alignment model. In the second part of my thesis I address secondary anisotropies of the cosmic microwave background: weak gravitational lensing and the nonlinear integrated Sachs-Wolfe (iSW) effect. The characteristic imprint of lensing can be used to reconstruct the lensing potential power spectrum. I show how this reconstruction is biased in the presence of primordial non-Gaussianities. For current values of f_NL, however, the bias is completely negligible on all but the largest angular scales. Finally, a novel analytical approach for the computation of the nonlinear iSW effect valid in the translinear regime is presented. It allows to identify two distinct contributions: the change of the gravitational self-energy density of the large-scale structure with (conformal) time and the Birkinshaw-Gull effect.
Fredrik Windmark
Planetesimal formation by dust coagulation (thesis pdf, 5 MB) Supervisor:
Kees Dullemond (ITA)
Thesis abstract: When the protostellar nebula collapses to form a star, some of the gas and dust is left in the form of a protoplanetary disk. Exactly how the subsequent formation of planetesimals proceeds is still not fully understood, but the coagulation of the dust is believed to play a vital role. One of the main problems with this picture is that a number of barriers have been identified, at which bouncing, fragmentation and radial drift prevent the formation of large bodies. We have investigated via theoretical models how large dust grains can grow in the presence of these barriers. This was done by examining some of the many assumptions that are generally used in the dust evolution modeling. We implemented a realistic model for the outcome of dust collisions, and we also studied the effect of velocity distributions and particle clumping, as well as the fate of large dust grains that drift inwards towards the star. In this process, we identified a new channel for planetesimal formation, and describe the initial steps towards an inside-out formation model where we give a prediction of the size and spatial distribution of the first generation of planetesimals.
Karsten Dittrich
Numerical simulations of planetesimal formation in protoplanetary disks (thesis pdf, 7 MB) Supervisor:
Hubert Klahr (MPIA)
Thesis abstract: In the first step of planet formation micrometer-sized dust grains grow in a protoplanetary disk through collisional sticking. This growth becomes inefficient at several centimeters up to meters in size, depending on the distance to the star. The resulting agglomerates are concentrated by turbulence in the disk up to densities at which they fragment through self-gravity to 100km-sized planetesimals. In my PhD thesis I simulate the concentration of dust particles in the turbulent gas flow of protoplanetary disks. Here I treat the gas as a fluid and solve the magnetohydrodynamic equations with the Pencil code. Dust particles are simulated as non-collisional point particles, decoupled from the grid. At first I test the particle representation of the Pencil code by comparing a Rayleigh-Taylor instability (RTI) simulation of a dust-laden fluid with a classical two-layer fluid RTI simulation. Additionally I simulate the sedimentation of a dust clump in a fluid which can be compared with experiments. Further I study zonal flows and the resulting long-lived axisymmetric pressure bumps that are created in magnetorotational instability simulations. Zonal flows are described by annuli of gas rotating faster or slower than the pressure-supported Keplerian rotation. They are created by temporal and spacial variances in the magnetic pressure. In a convergence study I measured a typical radial size of 5 to 7 vertical gas pressure scale heights with a life time of up to 50 local orbits (T_{orb} = 2\pi\Omega^{-1}). Particles get captured by these pressure bumps. For dust particles with a friction time \tau_f \ge 0.1\Omega^{-1} I found concentrations that are some hundred times higher than initially. Larger particles (\tau_f \ge 0.5 \Omega^{-1}) reach densities 10,000 times higher than their initial densities, sufficient to trigger secondary instabilities such as the streaming instability and gravitational collapse. I study the streaming instability in a zonal flow environment in simulations of higher resolution including the back-reaction drag from particles to the gas. These simulations show that the axisymmetric pressure bumps can accumulate enough particles to trigger the streaming instability, even with small particles (\tau_f = 0.1 \Omega^{-1}). Allowing for self-gravity dust clumps form, yet they are not stable against tidal forces. This is due to the insufficient resolution here. For my last project I studied the final collapse of a spherical dust cloud with a much higher resolution than in the above simulations. In this study I investigate a dust cloud with an initial density ranging from Roche density \rho\Roche down to 10^{-3}\rho_Roche$. Dust spheres with 0.1\rho_Roche, like I typically get from large scale simulations, fragment to a swarm of bound objects with a size distribution that is comparable to the observed size distribution of asteroids.
Federica Capranico
Statistics of Intrinsic alignments and Weak Lensing (thesis pdf, 14 MB) Supervisor:
Bjoern-Malte Schaefer (ARI)
Thesis abstract: The content of this work is two-fold. In the first part we present a study on the contamination of the intrinsic alignments to weak lensing measurements in the future survey Euclid. On the grounds of the tidal torque theory, we have adopted from the literature two related prescriptions for modeling the intrinsic alignment signal and computed for both the resulting biases in the cosmological parameters. We find a slight discrepancy among the two models, which both significantly (up to > 3 sigma) contaminate the estimates for Omega_m and sigma_8. The other parameters h, ns and w appear less affected. In the second part we present results based on an innovative statistical approach, the extreme value statistics. We investigate up to which level the primordial non-Gaussianities parameters fNL and gNL inherited by the bi- and trispectra of the weak lensing convergence can be constrained by the most extreme values of the convergence field. We find constraints of the order of 102 for fNL and 105 for gNL if individual extreme values are considered, therefore sadly showing only a relatively weak constraining power.

Statistics of Intrinsic alignments and Weak Lensing (thesis pdf, 4 MB) Supervisor:
Bjoern-Malte Schaefer (ARI)
Thesis abstract: First, changes in galaxy ellipticity statistics induced by weak gravitational lensing are studied. Ellipticity correlations are altered by lensing deflection on multipoles l > 1000 by ~ 5% for the ellipticity E-modes and by ~ 30% for the B-modes, while a shallower survey would exhibit larger changes. The E/B-mode conversion is analyzed, the possibility of measuring correlations between different multipoles, induced by homogeneity breaking, is discussed. Although correlations generated by weak gravitational shear dominate, the shifting effect is significant on small angular scales, causing a number of interesting phenomena possibly observable by future surveys. Second subject of this work is the time evolution of intrinsic correlations of galaxy ellipticities due to peculiar galactic motion. The magnitude and scale on which peculiar motions affect the ellipticity correlation is studied. Correlations are decreased on large multipoles l> 1000, up to 10 per cent in the E-mode and up to 60 per cent in the B-mode spectrum at l> 3000. E/B-mode conversion is present but small. Observational significance is concluded for larger scales for surveys at lower redshifts. Another aspect of weak lensing statistics is explored in the context of Bayesian model comparison. Frequentist and Bayesian statistics are contrasted in the context of inflationary non-Gaussianity models.
Alessandra Grassi
Baryon acoustic oscillations and primordial non-Gaussianities with weak lensing (thesis pdf, 13 MB) Supervisor:
Bjoern-Malte Schaefer (ARI)
Thesis abstract: This work introduces two investigations on possible new weak lensing applications. In the first part, I present a study on the possibility of detecting baryon acoustic oscillations by means of 3d weak lensing (3dWL). Basing our analysis on a Fisher matrix approach, we quantify the uncertainty on inferring the amplitude of the power spectrum wiggles with 3dWL. Ultimately, we find that surveys like Euclid and DES should be able to detect, respectively, the first four and three oscillations, with errors reaching the 1% or 10% of the amplitude for the first two wiggles in the case of Euclid. The second part of this work focuses on the study of primordial non-Gaussianities with a classical weak lensing approach. We study inflationary bi- and trispectra, the strentgh of their signals, and the consequences of fitting data with a wrong type of bispectrum on the inferred on fNL. We conclude that contraints on fNL are not competitive with the ones from CMB, but nonetheless valuable in case of a scale-dependent fNL. Lastly, we quantify lensing ability to test the Suyama-Yamaguchi inequality, and ascertain that Euclid could give evidence in favour or against the inequality for large non-Gaussianity values (tauNL > 10^5 or fNL > 10^2).
Faviola Molina Hernandez
Baryon acoustic oscillations and primordial non-Gaussianities with weak lensing (thesis pdf, 4 MB) Supervisor:
Ralf Klessen (ITA)
Thesis abstract: Understanding molecular cloud formation is a major challenge in modern astrophysics. Although the improvements on computational power and novel astronomical instrumentation have allowed us to reach unprecedented accuracy, there are still many open questions. One key issue which helps us to understand the physics behind this problem is the correct comparison between numerical models and observations. Usually, the $^{12}$CO($J$=1-0) emission is considered to be a good tracer of the temperatures and structure of molecular clouds. However, it has been found that it may provide a biased picture of clouds, at best. In this thesis, we analyze a large set of numerical simulations with the aim of making direct comparison with observations. Using a 3D magneto-hydrodynamical simulation including time-dependent chemistry, we find that most of the CO is located at number densities greater than 100 cm$^{-3}$ and kinetic temperatures ($T_{\rm K}$) below 40 K, regardless of the mean number density ($n_0$), metallicity ($Z$) and UV radiation field strength (UV). Radiative transfer calculations are performed to analyze the $^{12}$CO($J$=1-0) rotational transition line intensity that comes out of the cloud. We then calculate the excitation temperature ($T_{\rm ex}$) considering theoretical and observational approaches and find that the gas is mostly sub-thermally excited, indicating that $T_{\rm ex}$ represents a lower limit of $T_{\rm K}$. $T_{\rm ex}$ is used for estimating the CO column density ($N_{\rm CO}$). Considering the full position-position-velocity spectrum for inferring $T_{\rm ex}$, instead of the usual way of using the maximum of the intensity along the line of sight, improves the estimates of $N_{\rm CO}$ by $\sim$30%. Besides, when a single Milky-Way like CO-to-H$_2$ conversion factor is assumed, the total inferred mass of H$_2$ is underestimated by a factor which typically increases from $\sim$0.1 to 1, as the product $n_0\times Z$ decreases, and/or UV becomes stronger. Moreover, we propose density variance--Mach number relations for supersonic, magnetized, turbulent gas, including an isothermal and non-isothermal equation of state. These analytical relations reproduce satisfactorily the measurements made on numerical simulations. We find that the magnetic field strength scales with density in a relationship of the kind $B\propto \rho^{\alpha}$ with $0\leq \alpha< 1/2$.
Paola Pinilla Ortiz
Testing models of dust evolution in protoplanetary disk with millimeter observations (thesis pdf, 22 MB) Supervisor:
Cornelis Dullemond (ITA)
Thesis abstract: The first steps of planet formation imply that dust grows from interstellar-like particles to planetesimals. Diferent physical processes play an important role in the dust evolution and its ability to coagulate and fragment, such as: radial drift, settling to the midplane and turbulent mixing. Observational evidences have shown that millimeter sized particles survive in the outer regions of protoplanetary disks in spite of the fragmentation destructive collisions and rapid inward migration of dust bodies, which are theoretically expected. The main goal of my thesis is to compare the state-of-the-art of gas and dust evolution models with current millimeter observations and look for the most favorable scenarios to have a good agreement between the two. Different cases have been explored: typical disks around Sun-like stars, lowmass disks as the ones around Brown Dwarfs and disk with inner gaps or holes known as transition disks. This work therefore brings new theoretical constraints to the planetesimal formation in young circumstellar disks, in direct link with the available observations.
Meiert Willem Grootes
(The Netherlands)
The Evolution of Spiral Galaxies in the Group Environment (thesis pdf, 11 MB) Supervisor:
Richard Tuffs (MPIK)
Thesis abstract: Although the process by which galaxies obtain the gas needed for star-formation is amongst the most fundamental processes related to the formation of baryonic structure in the universe, there is very little in the way of empirical evidence with which to constrain theoretical models. In particular, the postulated environmental dependencies of this process, although widely modeled, remain largely unconstrained. In this work, I present the first detailed, quantitative analysis of the environmental effects on the process of gas-fueling in which the relevant effects of the galaxy - intergalactic medium interaction have been isolated from other potential environmental in uences. In the context of this analysis, a new robust method for selecting morphologically defined samples of galaxies by photometric proxies is developed, as well a powerful new method for correcting the UV/optical emission of samples of spiral galaxies for the effects of attenuation by dust located in their disks. Combining these tools with the data from the GAMA survey, in particular the galaxy group catalog, I present a detailed analysis of the environmental dependencies of gas-fueling. The results obtained require a fundamental re-evaluation of the assumptions concerning the fueling of satellite galaxies and the effects of active galactic nuclei.
Anton Prosekin
Propagation and radiation of ultrarelativistic particles in magnetic fields in different astrophysical environments (thesis pdf, 2 MB) Supervisor:
Felix Aharonian (MPIK)
Thesis abstract: The thesis work presents the results of theoretical studies of different scenarios for the propagation and the radiation of ultrarelativistic particles depending on the environment determined by the magnetic and low energy radiation fields. First, using the analytical solution of Boltzmann equation in the small-angle approximation, we have accurately calculated the angular, energy, and time distributions of the ultrahigh energy protons, gamma rays produced by synchrotron radiation of secondary electrons and positrons, and secondary neutrinos from the source of cosmic rays embedded in the magnetized environment of the level of B ~ 10^(−9) G. The second part considers the scenario explaining TeV gamma radiation from distant blazars by secondary gamma rays produced by cosmic rays along the line of sight in the weak magnetic field of the level of B ~ 10^(−15) G. We have studied the possibility of detection of TeV radiation from blazars with redshifts greater than z = 1. Finally, the last chapter of the work is addressed to the radiation of charged particles in the extremely strong magnetic fields of compact objects such as pulsar and black hole. We have studied the synchrotron and curvature radiation regimes and transition between them showing the strong sensitivity of radiation spectra on the pitch angle.
Ervin Kafexhiu
Radiation signatures of nuclear reactions in very hot astrophysical plasmas (thesis pdf, 5 MB) Supervisor:
Felix Aharonian (MPIK)
Thesis abstract: The importance of nuclear reactions in low-density astrophysical plasmas with ion temperatures kT>1 MeV has been recognized for more than thirty years. However, the lack of comprehensive data banks of relevant nuclear reactions and the limited computational power did not allow detailed theoretical studies. In this thesis, using the publicly available code TALYS, I have built a large nuclear reaction network relevant for temperatures exceeding 1MeV. It contains about 270 nuclear species and include the calculation of gamma-ray emissivity due to different nuclear reactions. The pi0-mesons production are also included. An approach to calculate the gamma-ray spectra through p + p to pi0 channel for an arbitrary proton distribution is also proposed. The nuclear network is applied to two-temperature accretion disk models the so-called Advection Dominated Accretion Flows (ADAF) and Shapiro-Lightman-Eardly (SLE). The gamma-rays emissivity are calculated for a wide parameter space including initial chemical composition. For a 10 solar mass black hole, both models can produce nuclear gamma-ray lines luminosities as large as L_N ~ 10^{34} erg/s. SLE is not an effective source of pi0 photons, whereas ADAF luminosity can be as large as L_pi ~ 10^{35} erg/s. ADAF regime is hot enough to evaporate neutrons. They can reach the companion star atmosphere and initiate secondary nuclear reactions.
Mathias Jaeger
Evolution of galaxies studied by the COMBO-17+4 survey (thesis pdf, 9MB) Supervisor:
Klaus Meisenheimer (MPIA)
Thesis abstract: In this thesis the evolution of galaxies at redshift z < 2 was analyzed, using the COMBO-17+4 (Classifying Objects by Medium-Band Observations) survey. This survey used optical as well as near-infrared data from three different fields of view with a total area of 0.625 square degree. On the basis of an H band selected catalog the photometric redshifts, rest-frame colors and masses of 42,134 galaxies to a limiting magnitude of H = 21.7 mag were calculated. Using the color bimodality in (U280 −V) the galaxies were divided in a red (passive) and a blue (active) population, which were analyzed separately. The results verify the existence of a bimodality up to a redshift of z = 1.6. Further the evolution of the luminosity function for both galaxy populations was analyzed in ten redshift bins. In both populations a significant decrease in the characteristic magnitude M∗B from z = 2 to z = 0.2 was measured. Moreover the number density of red galaxies showed a slight increase during this time, whereas the number density of blue galaxies was decreasing. An analysis of the mass function showed a constant value of the characteristic mass for both populations and an increase of the mass density for red galaxies from z = 2 to z = 0.2.
Chia-Chun Lu
An advanced direction reconstruction technique and application to the observation with H.E.S.S. (thesis pdf, 18MB) Supervisor:
Werner Hofmann (MPIK)
Thesis abstract: The High Energy Stereoscopic system (H.E.S.S.) is an array of five imaging atmospheric Cerenkov telescopes which aims at exploring the high energy non-thermal processes in the universe via detecting very high-energy (VHE) gamma rays. In this work, an advanced direction reconstruction algorithm which yields better point spread function (PSF) and sensitivity is developed to cope with the complex morphology analysis in the Large Magellanic Cloud (LMC) region. LMC is a satellite galaxy of Milky Way at the distance of 48 kpc. The relatively short distance makes it a good extra-galactic laboratory for astronomical observation. The H.E.S.S. observation focuses on the three targets hosted in this region: the young supernova remnant SN1987a, the pulsar wind nebula N 157B, and the superbubble 30Dor C. The VHE flux of SN 1987a is predicted at the detectable level for H.E.S.S. but no significant detection is found in the current dataset. An upper limit on the gamma-ray flux is derived for this target. The pulsar wind nebular N 157B is detected, and the spectrum and other physical quantities are derived. The gamma-ray flux shows that it is the most-energetic-ever observed pulsar wind nebula. At the vicinity of N 157B, we find extra gamma-ray excess towards the direction of 30Dor C. The existance of this source is established by detailed morphology studies and its connection to 30Dor C is discussed.
Jochen Tackenberg
Early stages of high-mass star formation (thesis pdf, 16MB) Supervisor:
Henrik Beuther (MPIA)
Thesis abstract: This thesis is dedicated to the formation of high-mass stars. In particular, we are interested in the initial conditions that allow massive stars to form. Therefore, we first examine the very initial stage of massive star formation. Based on a statistical survey study over 20 deg2 from mid-infrared to sub-millimeter wavelengths, we show that dense and massive starless clumps exist. Assuming that they will form stellar clusters instead of single objects, we estimate their lifetime to be between 15 kyr and 60 kyr. In order to understand such short lifetimes, we investigate the large-scale dynamical structure of high-mass star-forming regions. Using N2H+ observations we find smooth largescale velocity gradients in a number of filaments, which are consistent with gas flowing along the filament. Analyzing HCO+ and H13CO+ spectra, we find global supersonic gas infall onto a few regions. To understand whether the dynamics of an expanding Hii region can trigger star formation, we select a starless clump located on the rim of a bubble which is infrared-dark up to 160 μm. In a multi-wavelengths approach we search for imprints of the bubble on the gas. While an expanding Hii region might favor the formation of high-mass stars, we find no evidence that it triggers the collapse. In summary, this thesis emphasizes that high-mass star formation is a fast and dynamic process.
Ronald Läsker
Supermassive black holes and their host galaxies - fundamental scaling relations (thesis pdf, 128MB) Supervisor:
Glenn van de Ven (MPIA)
Thesis abstract: The research presented in this thesis aims at improving the characterization of the scaling relations of the masses of central Supermassive Black Holes with bulge and total luminosities of their host galaxies. These scaling relations are significant for our understanding of the evolution of galaxies and the origins of Supermassive Black Holes. As part of this investigation, 35 galaxies with known central Black Hole mass were observed at near-infrared wavelengths. The obtained images were subjected to a thorough photometric analysis, which led to reliable bulge and total luminosities. As a result, it could be shown that the slope of the correlation between central Black Hole masses and bulge luminosities was previously overestimated. Further, it was found that the correlation with total luminosity is equally tight as the correlation with bulge luminosity. A linear regression method was devolped that enables extraction of more detailed information about the cosmic scatter in the scaling relations. In the last part of this thesis, a Black Hole mass was determined by means of dynamical modeling. The measured mass far exceeds the prediction from current scaling relations, thereby putting the universality of the scaling relations in question and principally opening up opportunities to better understand the physics of galaxy and Black Hole formation.
Paul Boley
High-resolution studies of circumstellar material around massive young stellar objects (thesis pdf, 3MB) Supervisor:
Thomas Henning (MPIA)
Thesis abstract: In this thesis, I investigate the nature of circumstellar material around massive young stellar objects (MYSOs). I make extensive use of spatially-resolved observations, and examine spatial structure at scales ranging from tens of astronomical units up to nearly a parsec. In the first of three scientific studies presented in this thesis, I present the results of a survey to observe MYSO candidates with the Very Large Telescope Interferometer at mid-infrared wavelengths, using the MIDI instrument. I consider the properties of this sample of 20 objects as a whole, and address the sources individually. In the second study, I focus on the well-known MYSO IRAS 13481-6124. By combining interferometric data from both near- and mid-infrared wavelengths, I derive a temperature profile for the circumstellar disk around this object, which I discuss in the context of theoretical disk models. In the third study, I examine the high-luminosity source AFGL 4176 at wavelengths of 1 mkm–1 mm. I construct one-dimensional radiative transfer models of the envelope, and interpret the interferometric observations in terms of a parameterized disk model. The observations and results presented in this thesis will be of use to future studies of MYSOs, both theoretical and observational, and provide an important groundwork for observations of such objects with the next generation of interferometric instruments at the Very Large Telescope.
Gustavo Dopcke
On the physics of the low metallicity IMF (thesis pdf, 3MB) Supervisor:
Ralf Klessen (ITA)
Thesis abstract: A fundamental question in the context of structure formation concerns the transition from massive primordial stars to Population I/II stars with typical masses of less than 1 M_Sol, and in particular the physical mechanisms inducing this transition. The purpose of this work is to study the physical mechanisms that shape the stellar IMF at the early stages of the Universe. With this aim, we perform a set of eight hydrodynamic simulations that include sink particles representing contracting protostars. Evidence for fragmentation was found in all cases, and hence we conclude that there is no critical metallicity below which fragmentation is impossible. Nevertheless, there is a clear change in the characteristic mass of the clouds at Z = 10^-4 Z_Sol. Moreover, we develop a model for the stellar IMF, which accounts for the evolution of simulated clusters. With that, we can recover results from hydrodynamic simulations, and also observations of the Orion Nebulae Cluster. Finally, we find that the resultant stellar mass distribution is highly dependent on the characteristic mass of the cluster, and the mode of mass accretion.
Milica Micic
Molecular cloud formation with self-consistent thermal and chemical treatment of the gas (thesis pdf, 5MB) Supervisor:
Ralf Klessen (ITA)
Thesis abstract: Understanding physical and chemical processes that guide the formation and evolution of giant molecular clouds (GMCs) has important implications for the formation of stars. GMCs dominantly consist of molecular hydrogen, but there are more than 200 chemical species of various combinations of carbon, nitrogen and oxygen atoms. Together, these species control the cooling ability with the thermal and dynamical evolution of the gas cloud. In order to overcome the restrictions encountered by most previous models of molecular cloud formation due to the complexity of chemical reaction networks and its inclusion in hydrodynamical codes, we have implemented detailed treatment of atomic/molecular cooling and hydrogen chemistry into state-of-art high resolution hydrodynamical simulations. The main focus of our study is on the influence that choosing between different cooling functions and turbulent driving has on the formation and evolution of molecular gas. In that manner, we study the influence of the nature of the turbulence on the formation of molecular hydrogen by examining both solenoidal (divergence-free) and compressive (curl-free) turbulent driving. The obtained results we use to test a simple prescription suggested by Gnedin et al. (2009) for modelling the influence of unresolved density fluctuations on the H2 formation rate in largescale simulations of the ISM. We also investigate the properties of the dense clumps formed within our model of the molecular cloud formation in converging flows and directly compare the results obtained using the simple, parametrized cooling function introduced by Koyama & Inutsuka (2002) and used by a number of converging flows studies with the results of the detailed calculation of the non-equilibrium chemistry and thermal balance of the gas. Finally, we study C I and CO emission from molecular clouds in comparison to their column densities and the total column density, as we look for the way to trace the structure of the cloud.
Ana Valente
On the cross-correlation between weak gravitational lensing and the Sunyaev-Zel'dovich effect (thesis pdf, 11MB) Supervisor:
Matthias Bartelmann (ITA)
Thesis abstract: The distribution and evolution of cosmic structures, in their dark and baryonic components, remains a source of uncertainty in the current model of the Universe and is an active field of research. To address this subject, we cross-correlate the weak gravitational lensing and thermal Sunyaev-Zel’dovich effects of galaxy clusters as a function of redshift. We use the halo model of large-scale structure to estimate the two-point correlation function between the dark matter density and the gas pressure in halos. After obtaining the three-dimensional power spectrum and projecting it onto the sky by means of Limber’s approximation, we estimate how the cross-correlation between these two types of data builds up as redshift increases. We calculate the covariance matrix for a measured cross-correlation function and evaluate expected error bars for realistic surveys. Further, we examine how the choice of cosmological parameters affects our results and inspect the influence of cluster properties on the cross-correlation signal. We find that, although the cross-correlation signal does not seem to be suitable for constraining cosmological parameters, it is highly sensitive to the intrinsic properties of the clusters and thus provides a way to characterise the evolution of the halo gas component.
Dading Nugroho
Star formation properties and kinematics of nearby AGN host galaxies (thesis pdf, 11MB) Supervisor:
Knud Jahnke (MPIA)
Thesis abstract: We investigate the spatially resolved kinematics properties of type 1 quasar host galaxies with redshift z < 0.2. Using combination of kinematic analysis with morphological information from optical imaging data, we diagnose the existence of disturbances in velocity field and morphology to study the importance of major merger to ignite quasar activity. We present results of 19 quasar host galaxies analysis observed with VIMOS integral field spectrograph. We construct a quantitative distortion classification scheme for the velocity fields and we demonstrate that the occurrences of distortion in the velocity fields and optical imaging are generally consistent with each other. The prevalence of mild level of distortion signatures indicates that majority of our quasars are not in the most intense part of galaxies merger process. Further, we investigate a possible relation between interaction state of the host galaxy and level of supermassive black hole activity. Using our sample, we find there is no dependency of accretion rate on neither black hole mass nor distortion state of the host galaxy and is therefore unable to support the hypothesis that major merger is the most dominant fueling mechanism to ignite quasar activity.
Alumni - 2012
Daniel Seifried
Magnetic fields during the early phase of massive star formation (thesis pdf, 9MB) Supervisor:
Robi Banerjee (ITA)
Thesis abstract: The goal of this work is to improve our current understanding of the formation process of massive stars in the presence of magnetic fields by means of numerical simulations. In particular, I focus on protostellar accretion rates, the evolution and the properties of protostellar discs and their associated outflows, and the interplay of turbulence and magnetic fields and its impact on protostellar disc formation. In a systematic parameter study I show that the accretion rates are remarkably constant over a wide range of initial conditions. Furthermore, I show that in the absence of turbulence for strong initial magnetic fields only sub-Keplerian discs can form which is attributed to the strong magnetic braking effect. This result seems to be in contrast to observational results. The morphology of the outflows, which shows a strong dependence on the initial conditions, can ultimately be linked to the structure of the underlying disc. Well-collimated outflows with high outflows velocities only develop if a Keplerian protostellar disc is present, otherwise slowly expanding, sphere-like outflows develop. Furthermore, I analyse the driving mechanism of outflows with an analytical criterion derived in the course of this work. When including supersonic, turbulent motions in the simulations, Keplerian protostellar discs form in contrast to the non-turbulent simulations. This result is in agreement with observations of early-type protostellar objects.
Hagen Meyer
The progenitors of early-type dwarf galaxies in the Virgo cluster (thesis pdf, 11MB) Supervisor:
Thorsten Lisker (ARI)
Thesis abstract: In the here presented thesis, I investigate the possible progenitors of elliptical dwarf galaxies in the Virgo galaxy cluster. For the analysis of the galaxies of the late morphological type, a broadband data set in the u, g, r, i and z filters of the Sloan Digital Sky Survey was used. The photometric and structural properties (e.g. luminosity and radius) of the single morphological subclasses was compared with an existing data set of elliptical dwarf galaxies to explore the possible evolutionary connections. Within the sample, galaxies of the type "blue compact dwarf" - if the contribution of the starburst component is removed - show a remarkably good similarity to the elliptical dwarf galaxies. This agreement in the photometric properties could point to a possible evolutionary connection. The results of the photometry of the entire sample were used as input parameters for an evolutionary synthesis code, which simulates the undisturbed and disturbed evolution of the photometric parameters of the galaxies. By comparing the results of the simulation with the ones of the observations of today's elliptical dwarf galaxies, I am able to restrict the possible progenitors to certain morphological subclasses. In the course of this thesis, I was able to show that apart from "blue compact dwarf galaxies", also the prospective photometric properties of late-type spirals of the type Sd and Sm are in good agreement with today's elliptical dwarf galaxies.
Eva Lefa
Non-thermal radiation processes in relativistic outflows from AGN (thesis pdf, 2MB) Supervisor:
Felix Aharonian / Frank Rieger (MPIK)
Thesis abstract: Non-thermal, leptonic radiation processes have been extensively studied for the interpretation of the observed radiation from jets of Active Galactic Nuclei (AGN). This work addresses the synchrotron and Inverse Compton scattering (ICS) mechanisms, and investigates the potential of a self-consistent, time-dependent approach to currently unsolved problems. Furthermore, it examines how deviations from standard, one-zone models can modify the radiated spectrum. A detailed analysis of the shape of the ICS spectrum is also performed. In the first part a possible interpretation of the hard γ-ray blazar spectra in the framework of leptonic models is investigated. It is demonstrated that hard γ-ray spectra can be generated and maintained in the presence of energy losses, under the basic assumption of a narrow electron energy distribution (EED). Broader spectra can also be modeled if multiple zones contribute to the emission. In such a scheme, hard flaring events, like the one in Mkn 501 in 2009, can be successfully interpreted within a "leading blob" scenario, when one or few zones of emission become dominant. In the second part the shape of the Compton spectrum close to the maximum cutoff is investigated. Analytical approximations for the spectral shape in the cutoff region are derived for various soft photon fields, providing a direct link between the parent EED and the upscattered spectrum. Additionally, a generalization of the beaming pattern for various processes is derived, which accounts for non-stationary, anisotropic and non-homogeneous EEDs. It is shown that anisotropic EEDs may lead to radiated spectra substantially different from the isotropic case. Finally, a self-consistent, non-homogeneous model describing the synchrotron emission from stratified jets is developed. It is found that transverse jet stratification leads to characteristic features in the emitted spectrum different to expectations in homogeneous models.
Iwona Mochol
Nonlinear waves in Poynting-flux dominated outflows (thesis pdf, 2MB) Supervisor:
John Kirk (MPIK)
Thesis abstract: Rotating, compact objects power some of the most spectacular phenomena in astrophysics, e.g., gamma-ray bursts, active galactic nuclei and pulsar winds. The energy is carried by Poynting flux, and the system is usually modelled using relativistic magnetohydrodynamics (MHD). However, in the relatively low density medium expected around some of these objects, the MHD approximation breaks down, allowing new, large-amplitude waves to propagate. We discuss the role of these waves in two astrophysical contexts: In blazar jets, we show that a magnetic shear, launched together with a plasma from the black hole magnetosphere, begins to accelerate particles at a large distance from its source. The resulting non-thermal emission can, nevertheless, be modulated on very short timescales, which can explain the rapid variability of the TeV gamma-ray flux observed from some blazars. In pulsar winds, we analyze the radial propagation of superluminal modes, including their damping by radiation reaction and by interaction with an external photon field. We discuss their effect on the structure of the pulsar wind termination shock, presenting new solutions in which the non-linear wave is asymptotically matched to the constant pressure surroundings. The observational implications of these solutions are discussed for both isolated pulsars, and pulsars in binary systems.
Tessel van der Laan
(The Netherlands)
Circumnuclear star forming rings in the barred galaxies NGC5248 and NGC6951 (thesis pdf, 11MB) Supervisor:
Eva Schinnerer / Hans-Walter Rix (MPIA)
Thesis abstract: In this thesis I study the gas distribution and dynamics, as well as the stellar populations, in the circumnuclear star forming rings in the nearby barred galaxies NGC5248 and NGC6951. Circumnuclear star forming rings are regions of high gas and star formation density in the centers of disk galaxies. These star formation rings can have a strong influence on the secular evolution of their host galaxies, by halting gas inflow towards the nucleus. The gas masses that build up in these rings are high and this consequently leads to intense star formation. This makes circumnuclear star forming rings also an important tool to build up the stellar (pseudo-)bulge. These two galaxies were selected because they have very similar global properties (mass, size, metallicity), but very different circumnuclear regions, both of which containing circumnuclear star forming rings. Detailed investigation of both, and a comparison between them, thus provides new detailed insight on the interplay between circumnuclear star forming rings and the circumnuclear region as a whole. Three questions are addressed in this work. First of all, I investigate the true effectiveness of the circumnuclear rings as a gas barrier in both galaxies. I study star formation progression in these rings and its relation to the gas flow onto the ring. The age and life span of the circumnuclear rings is derived from the analysis of their stellar populations. In both galaxies I find that while the circumnuclear rings are effective barriers, they are not absolute ones. The interplay between the large scale stellar bar, circumnuclear ring and other dynamical components in the circumnuclear region allows some gas to flow towards the very center past the ring. I find that the location of the youngest stars in the ring is close to the inflow position of the gas onto the ring in both galaxies, consistent with star formation proceeding in a ’pearls-on-a-string’ mechanism with the youngest ’pearls’ close to the gas inflow location. Finally, I derive ages of the rings around 1.5 Gyr, which implies that circumnuclear star forming rings have a significant amount of time to shape the circumnuclear regions of their host galaxies.
Natalie Miriam Raettig
Global Baroclinic Instability and its Implications on Planet Formation (thesis pdf, 60MB) Supervisor:
Hubert Klahr (MPIA)
Thesis abstract: In this thesis we analyze a form of non magneto hydrodynamic turbulence which could be described as disk weather since it forms vortices due to baroclinic effects. We want to find out if and how these vortices influences planet formation. The focus is on angular momentum transport and how efficient vortices can trap particles. We estimate disk properties from observations and derive radial Brunt-Väisälä frequencies as well as cooling time-scales. Then we analyze the baroclinic amplification of vortices and the particle concentration therein. We use 2D as well as 3D local shear- ing box simulations with the PENCIL CODE to investigate the problems. In 2D, we conduct a comprehensive study of a broad range of various entropy gradients, thermal cooling and thermal relaxation times covering the parameter space relevant for protoplanetary disks. We measure the Reynolds stresses as a function of our control parameters and see that there is angular momentum transport even for entropy gradients as low as beta equiv −d ln S/d ln r = 1/2. The amplification-rate of the perturbations appears to be proportional to beta^2. The saturation level of Reynolds stresses on the other hand seems to be proportional to beta^1/2. All entropy gradients will lead to Reynolds stresses of 10^−3 − 10^−2 which shows that baroclinic vortices are a feasible mechanism for transporting angular momentum. The concentration of particles of different sizes in baroclinic vortices is first analyzed in 2 dimensions. Because we expect strong particle accumulations, particle feedback onto the gas is included. Particles accumulate inside the vortices and the local dust-to-gas ratios become high enough to trigger the streaming instability even for initial dust-to-gas ratios as low as epsilon_0 = 10^−4. In 3 dimensional unstratified gas simulations we verify previous result. Once particles, that feel vertical gravity, with normalized friction times of St = 0.05 and St = 1.0, and epsilon_0 = 0.01 are included, the vortex column in the mid-plane is strongly perturbed. Yet, when the initial dust-to-gas ratio is decreased the vortices remain stable and function as efficient particle traps. Streaming instability is triggered even for the lowest epsilon_0 = 10^−4 and smallest particle sizes (St = 0.05) we assumed, showing a path for planetesimal formation in vortex cores from even low global amounts of cm-sized particles.
Natalia Sergeevna Kudryavtseva
Micro-arcsecond astrometry of exoplanet host stars and starburst clusters (thesis pdf, 4MB) Supervisors:
Wolfgang Brandner / Stefan Hippler (MPIA)
Thesis abstract: Since the first star catalogues tremendous progress in the astrometric accuracy of positional observations has been achieved. In this thesis, I show how beneficial astrometric techniques are already today for the study of starburst clusters, and how astrometry will fundamentally improve our knowledge on exoplanets in the near future. I first study two galactic starburst clusters, Westerlund 1 (Wd 1) and NGC 3603 YC, which are among the most massive young clusters in our Galaxy. I perform astrometric and photometric analyses of adaptive optics and Hubble Space Telescope observations of these clusters in order to understand on which time-scales these clusters formed. As a result, I derive upper limits for the age spreads of 0.4 Myr for the 4 to 5 Myr old cluster Wd 1, and 0.1 Myr for the 1 to 2 Myr old NGC 3603 YC. Thus, the star formation process in each of these clusters happened almost instantaneously. The second part of this thesis deals with the dynamical properties and the initial mass function (IMF) of Wd 1. Astrometric analysis of multi-epoch, near-infrared adaptive optics observations of Wd 1 was used to distinguish the cluster's members from field stars. This lead to an unbiased determination of the internal velocity dispersion of the cluster, and an IMF slope of Gamma = -0.46 for the core of the cluster (R < 0.23 pc). The final part of this thesis is devoted to the future prospects of detecting exoplanets with the GRAVITY instrument. The second-generation Very Large Telescope Interferometer instrument GRAVITY aims at achieving 10 μas accuracy. Here, I discuss the possibilities of detecting and characterizing exoplanets with masses down to a few Earth masses with the high sensitivity provided by GRAVITY, in addition to providing an initial target list.
Oleksiy Golubov
Modelling the Milky Way Disc (thesis pdf, 4MB) Supervisor:
Andreas Just (ARI)
Thesis abstract: RAVE, SEGUE and Hipparcos data are used to study the dynamics of stars in the extended solar neighbourhood. The asymmetric drift of thin disc dwarfs is studied as a function of colour and metallicity. Linear extrapolation of the data falls within the error bars from Aumer & Binney (2009) for local standard of rest. The observed metallicity dependence of the asymmetric drift is consistent with the known radial metallicity distribution in the disc. Implying the asymmetric drift correction to the SEGUE data allows us to reconstruct the behaviour of the rotation curve of the Milky Way in the extended solar neighbourhood. The rotation curve appears to be eßentially flat, giving no hint for a dip just outside the solar radius followed by an increase observed in some other data sets. The data are supplemented by tangent point measurements for the inner rotation curve. Thus a synthetic rotation curve of the Milky Way is obtained. It is fitted by a density model consisting of a Dehnen bulge, an exponential disc with a hole, and a flattened dark matter halo with either cored isothermal or NFW density profile. In this fitting the parameters are constrained to reproduce the local surface density of the disc and the local volume density of the halo, which are known from local stellar dynamics in the solar neighbourhood. Thus the density model of the Milky Way is reconstructed. The vertical structure of the disc of theMilkyWay is consistent with the model by Just & Jahreiß (2010). Some basic features of distribution functions of the Milky Way and of the dynamical heating are also discussed.
Kasper Borello Schmidt
Frontiers of Galaxy Evolution: Time-Domain Observations and 3D Spectroscopy (thesis pdf, 10MB) Supervisor:
Hans-Walter Rix (MPIA)
Thesis abstract: Understanding the formation and evolution of galaxies through cosmic time has been a central focus of astrophysics in the last decades: how did the interplay between dark matter structure formation, star formation, galaxy merging, and active galactic nuclei (AGN) give rise to the observed galaxy properties at different redshifts? This thesis presents innovative observational approaches to two aspects of this problem: finding and studying AGN through their variability, and making a first systematic census of galaxy mergers at z > 1 through three-dimensional spectroscopy.
First we present a new and simple technique for selecting extensive, complete, and pure quasar samples via their intrinsic variability, parameterizing the single-band light-curve structure function through a power-law to identify quasars among other variable and non-variable sources. Using extensive multi-epoch observations from SDSS Stripe 82 containing ~60 epochs taken over 8 years, we demonstrate the power of this approach. The presented algorithm identifies quasars with a completeness and purity above 90% at all redshifts. Even for Pan-STARRS 1 mock data of only 6 epochs over 3 years, variability is still an encouragingly efficiencycient quasar classifier. Data on intrinsic quasar variability enable a wide range of astrophysical science. We quantify the color variability, confirming and greatly fleshing out previous claims, that quasars become bluer as they brighten. We find a strong redshift dependence of this blueing, which we can attribute to emission lines contributing to the SDSS bands at different redshifts. We find that the color variations of single quasars are much more pronounced than the ranges in color seen in time-averaged ensembles of quasars. This indicates, that the observed color variations cannot be explained by changes in the mean steady state AGN accretion rate, but must arise from accretion disk ‘hotspots’ or similar phenomena.
In the second, distinct part of the thesis, we present the first large sample of morphologically selected galaxy mergers with three-dimensional spectroscopy at z~1.5. With individual masses and star formation rates derived from multi-band photometry, we created emission line maps from the slitless grism spectroscopy of 3D-HST as proxy for star formation maps, providing a comprehensive empirical picture of where star formation takes place in galaxy mergers at the epoch, where the cosmic star formation and merger rates peaked. We find that a broad range of star formation morphologies occur at all redshifts, irrespective of star formation rate and total stellar mass, in these mergers. An initial illustrative comparison to a set of cosmological simulations shows, that simulated mergers with similar mass and gas content show star formation in both merger components far more often than for the observed 3D-HST mergers. This suggests that mergers at z~1.5 most commonly happen between galaxies of distinctly different gas fractions.
Philipp Girichidis
Importance of the initial conditions for star formation (thesis pdf, 10MB) Supervisor:
Robi Banerjee (ITA)
Thesis abstract: This thesis investigates the impact of the initial conditions on present-day star formation. Using numerical simulations, we follow the gravitational collapse of dense molecular clouds under different initial turbulent motions and initial density distributions. Our analysis focuses on the morphology of the cloud, the time and location of the formation of stars, the energetics during the collapse, the formation of clusters including their internal structure, their accretion behaviour as well as their mass distribution. The morphology of the cloud and the total number of stars are strongly influenced by the initial type of turbulence and the initial density profile. The results range from almost unperturbed cores with a single star to strongly filamentary cores with hundreds of stars in disconnected clusters. The internal structure of protostellar clusters is systematically but not significantly influenced by the initial conditions. Concerning the accretion rates as well as the dynamical interactions of stars within the clusters, we observe a fairly uniform behaviour, not reflecting the large variations in the initial conditions. The simulations presented in this thesis were performed using the grid-based code FLASH, developed mainly at the University of Chicago.
Xiaoying Pang
A comprehensive study of the young star cluster HD 97950 in NGC 3603 (thesis pdf, 15MB) Supervisor:
Eva Grebel (ARI)
Thesis abstract: I study the young massive star cluster HD,97950 located in the Galactic giant HII region NGC,3603. My goals are (1) to estimate the survival probability of the cluster, (2) to investigate the origin of its mass segregation, and (3) to investigate the interplay between the cluster and the surrounding interstellar medium (ISM). All the studies are done with data of the Hubble Space Telescope. I determine the cluster velocity dispersion from the stars' relative proper motions, and calculate the virial mass of the cluster. The cluster star formation efficiency is estimated to be about 50%, which suggests that the HD97950 cluster will most likely survive as a bound cluster to gas expulsion. I apply the Lambda minimum spanning tree technique to measure the mass segregation down to 30,Msun. The high-mass stars are more segregated than low-mass stars, implying that the mass segregation in HD97950 is mostly of dynamical origin. To improve the age determination for the cluster stars that are severely reddened by the surrounding dusty ISM, I compute a pixel-to-pixel distribution of the gas reddening, E(B-V)g, associated with the cluster. The radial profiles of E(B-V)g show significant spatial variations around HD97950. Using UBVRI photometry, I estimate the stellar reddening of cluster stars. After correcting for foreground reddening, the total to selective extinction ratio in the cluster is RV=3.49 pm 0.79. The extinction curve in the UBVRI filters in the cluster is greyer than the average Galactic extinction laws, but close to the extinction law for starburst galaxies. This indicates that stellar feedback from massive stars changes the dust properties in the HD,97950 cluster in a similar way as in starburst galaxies.
Raoul Haschke
Structure, Abundances, and Reddening of the Magellanic Clouds from its variable stars (thesis pdf, 40MB) Supervisor:
Eva Grebel (ARI)
Thesis abstract: Using the data obtained by the Optical Gravitational Lensing Experiment (OGLE) we investigate the properties of the old population of the Magellanic Clouds (MCs) using the variable RRLyrae stars and of the young population traced by Cepheids. With our own reddening estimates, we deduce distances to the RR Lyrae stars and Cepheids. These distances are used to determine structural parameters, such as scale height, position angle and inclination of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). The depth extent of the SMC is significantly higher than for the LMC. For the SMC a quite different orientation of the young Cepheids is found in comparison to the old RR Lyrae stars. One of the most interesting results is the off-center bar of the LMC, which extends for up to ∼ 5 kpc in both populations out of the disk. We determine metallicities for single RRLyrae stars and the metallicity distribution functions reveal significant differences between the LMC and SMC. The mean metallicity of the SMC is ∼ 0.2 dex lower. This leads to the conclusion of a different early enrichment history of the MCs. Investigating RRLyrae stars in the OGLE field towards the Galactic bulge, we obtain reddening estimates, metallicities and distances to the bulge and to a small part of the Sagittarius stream. The stream has a significantly lower metallicity and seems to be inclined. Performing a spectroscopic analysis of individual element abundances of nine very metal-poor candidate RRLyrae stars, we find the most metal-poor star ([Fe/H] = −2.7 dex) yet detected in theMCs. The other very metal-poor stars have abundances leading to the conclusion that a build-up of the Galactic halo from a merger with a MC-type galaxy is plausible.
Ana Lucia Uribe Uribe
The migration of planets in protoplanetary disks (thesis pdf, 15MB) Supervisor:
Hubert Klahr / Thomas Henning (MPIA)
Thesis abstract: This thesis presents a numerical study on the interaction between planets and circumstellar disks. We use the hydrodynamics/magnetohydrodynamics code PLUTO (Mignone et al, 2007) to simulate the circustellar accretion disk. A module to include embedded planets was incorporated into the code. We study two critical aspects for planet formation theory: the migration of planets due to gravitational disk torques and the accretion of gas onto planets from the surrounding disk. These two aspects are critical in any planet formation model as they will determine the final mass and the orbital separation. We first investigate aspects for massive planets in the evolutionary phase when a gap has been cleared in the disk. It is found that when a gap has been opened, the migration and gas accretion rate is linearly dependent on the surface density inside the gap. The torques exerted on the planet depend strongly on the material inside the Hill sphere when the local disk mass exceeds the planet mass. The depletion of the Hill sphere due to an accreting planet can increase migration timescales up to an order of magnitude of the linear estimate. Secondly, we investigate migration and gas accretion in turbulent disks, where turbulence is generated by the magnetorotational instability (MRI). In weakly magnetized turbulent disks, low-mass planet migration is dominated by stochastic density perturbations that can be characterized with a given amplitude and correlation time. More massive planets can undergo slower or reversed migration due to unsaturation of the corotation torque by turbulent advection and diffusion of gas into the horseshoe region. Magnetic turbulence is greatly supressed by giant planets that open a gap in the disk. Additionally, Jupiter-mass planets in turbulent disks are found to accrete less than expected from the global-averaged interal stresses in the disk. Our results can be directly implemented in planet population synthesis studies in order to better understand the nature of the observed population of extrasolar planets.
Alexander Hansson
The dependence of present-day galaxy properties on environmentand mass (thesis pdf, 17MB) Supervisor:
Thorsten Lisker (ARI)
Thesis abstract: In this thesis I show that only some of the commonly used stellar populations models can account for observed broad-band colors of the local galaxy population. Through a stellar population synthesis modeling, including the ects of dust, I show that a galaxy's star formation history, stellar mass, chemical enrichment and dust extinction can be constrained over a large part of the parameter space using optical broad band photometry. An essientially volume complete sample of nearby (<50Mpc) galaxies in the Sloan Digital Sky Survey are selected and u,g,r,i,z-band photometry for these galaxies are measured from the survey images. Using my stellar population synthesis modeling I obtain the aforementioned properties and study how these, along with galaxy sizes and shapes, depend on galaxy stellar mass and environment. In addition, these galaxy properties are related to the mass functions of galaxy groups providing further insight into galaxy assembly. Moreover, I have performed an imaging study of the Abell 569 cluster core revealing signs of galaxy interactions as well as a galaxy luminosity function with a at faint end slope.
Fazeel Mahmood Khan
Dynamics and Evolution of Supermassive Black Holes in Merging Galaxies (thesis pdf, 12MB) Supervisor:
Andreas Just (ARI)
Thesis abstract: Supermassive black holes (SMBHs) are ubiquitous in galaxy centers and are correlated with their hosts in fundamental ways, suggesting an intimate link between SMBH and galaxy evolution. In the paradigm of hierarchical galaxy formation this correlation demands prompt coalescence of SMBH binaries, presumably due to dynamical friction, interaction of stars and gas with the binary and finally due to gravitational wave emission. If they are able to coalesce in less than a Hubble time, SMBH binaries will be a promising source of gravitational waves for gravitational wave detectors. However, it has been suggested that SMBH binaries may stall at a separation of 1 parsec. This stalling is sometimes referred to as the Final Parsec Problem (FPP). This study uses N-body simulations to test an improved formula for the orbital decay of SMBHs due to dynamical friction. Using a large set of N-body simulations, we show that the FPP does not occur in galaxies formed via mergers. The non spherical shape of the merger remnants ensures a constant supply of stars for the binary to interact with. On its way to coalescence, the SMBH binary ejects several times its total mass in stars and leads to the formation of the cores observed at the center of giant ellipticals. The results of this study also support a cosmological scenario where the prompt coalescence of SMBHs following galaxy mergers is common and where SMBH binaries are promising sources of gravitational waves at low and high redshifts.
Alumni - 2011
Gisella de Rosa
A comprehensive analysis of optical and near-infrared spectroscopy of z~6 quasars (thesis pdf, 5MB) Supervisors:
Fabian Walter / Hans-Walter Rix (MPIA)
Thesis abstract: High redshift (z~6) quasars (QSOs) are unique probes of the early growth of accreting supermassive black holes (BHs) and are fundamental tools to study the ionization state of the intergalactic medium and its chemical enrichment. Until now, only the most luminous QSOs have been studied, often one object at the time. In this thesis we present the most extended consistent analysis to date of optical and near-infrared spectra of z~6 QSOs. Our NIR sample is composed of 22 spectra of QSOs at 4.5 An analysis of the wavelength shifts of the optical lines shows that the redshifts inferred from the OI+SiII complex, visible in most of the optical spectra, are consistent with those obtained from the MgII line. This implies that zOI is a reliable proxy for the QSO systemic redshift. We create a composite template of the z~6 QSOs that, red-ward the Ly-alpha line, remarkably resembles the one obtained for low-redshift QSOs. Investigating the existence of possible correlations amongst optical emission properties, we do not detect the expected anti-correlation between the EW and the QSO continuum luminosity (Baldwin effect) for the high ionization lines (NV & SiIV+OIV]). We show that a strong correlation holds between the line FWHM and its luminosity. Given the narrow distribution of Eddington ratios that characterizes the z~6 QSOs population, this implies that QSOs with brighter lines are the ones that host more massive BHs. We further confirm this statement by testing the relation between the line luminosity and MBH.
Finally we use our optical spectra to study the ionization state of the IGM at high redshift, by measuring the evolution of the near zone (NZ) sizes and that of the Gunn Peterson (GP) effective optical depth. Our comprehensive analysis shows that the correlation between the NZ radii and the QSO systemic redshifts is significantly reduced with respect to previous results. This implies that changes of the IGM ionization state are difficult to constrain through such measurements. We also outline a new method to detect the evolution of the GP effective optical depth through the analysis of the Ly-alpha and Ly-beta flux ratios. For each QSO we build a model based on the SDSS QSO composite spectrum and on the optical depth parametrization of lower-redshift absorbers (z<5.7). Comparing the QSOs flux ratios with the ones obtained from the models we detect a steep increase in the evolution of the effective optical depth.
Christoph Deil
HESS and Fermi Surveys of the Galactic Gamma-ray Source Population (thesis pdf, 80MB) Supervisor:
Werner Hofmann (MPIK)
Thesis abstract: The High Energy Stereoscopic System (HESS) is an array of four imaging atmospheric Cherenkov telecopes, located in Namibia, observing the gamma-ray sky at energies > 100 GeV since 2004. For the first time HESS has surveyed the Galactic plane (approximately in the range GLON = -110 to +70 deg, GLAT = -3 to 3 deg) and detected a number of sources, each one a cosmic particle accelerator emitting gamma rays produced in interactions of the cosmic rays with ambient matter and radiation fields. In this thesis the full HESS Galactic plane survey dataset was used to derive significance and flux maps as well as a catalog of 62 sources containing their position, extension and spectrum. Several new methods for an improved and semi-automatic detection and analysis of all sources were developed. The Fermi Large Area Telescope (LAT) is a space-based gamma-ray telescope, continuously performing an all-sky survey above 100 MeV since June 2008. Based on two years of data in the energy range 100 MeV to 100 GeV the LAT collaboration has published a catalog of 1873 sources, 244 of which, mostly of Galactic origin, are located inside the HESS survey region. In this work allsky significance and flux maps and catalogs of 74 Fermi sources above 10 GeV and 42 sources above 100 GeV are derived and a preliminary comparison with the HESS data is presented. This work can serve as the basis for future detailed studies of the Galactic gamma-ray source population.
Carolina Bergfors
Formation of stars, substellar objects and exoplanets : Observations of multiplicity (thesis pdf, 2MB) Supervisors:
Wolfgang Brandner / Thomas Henning (MPIA)
Thesis abstract: Many nearby stars are part of a binary or multiple system. Details about their history are preserved in their multiplicity characteristics, and observations of binary/multiple star systems provide a way to measure fundamental physical properties of the stars as well as clues to their formation and evolution. Moreover, planet formation and dynamics may also be affected by the presence of a second star, or by giant planets in the same system.
In this thesis, high resolution imaging of low-mass stars, planet host stars and a multiple planet system is presented. The results of observations and analyses include the discovery of several previously unknown companion stars and multiplicity statistics for M dwarfs in the largest M dwarf multiplicity survey to date. We also present near-infrared characterization of four close M dwarf systems, previously unknown companion candidates to exoplanet host stars, and investigate how a close companion may affect planet formation. New astrometric data is presented for three of the directly imaged planets in the HR8799 system, and an analysis of a possible orbital configuration of planet HR 8799 d.
Mario Gennaro
Massive clusters revealed in the near infrared : Constraining the early stages of stellar evolution (thesis pdf, 25MB) Supervisors:
Wolfgang Brandner / Thomas Henning (MPIA)
Thesis abstract: Stellar clusters can be considered as the building blocks of galaxies. Studying how clusters form and evolve is crucial in understanding the evolution of their host galaxies. Young massive stellar clusters, in particular, play a key role in placing constraints on the physics of star formation and evolution. The stellar population inside these extremely massive objects spans the full range of stellar masses from very low mass brown dwarfs to the most massive stellar objects currently known. One of the most important tools to study stellar clusters are stellar evolutionary models, i.e. tracks and isochrones from which masses and ages can be derived. Resolved stellar populations in clusters can in turn be used to test and calibrate these theoretical models.
In this work I first present a study of the current generation of pre-main sequence models and their thorough use for assessing stellar properties. I then continue using stellar models to study the most massive young cluster in the Milky Way: Westerlund 1. The last part of this work deals with the first multiple wavelength investigation of a recently discovered star forming region: the CN15/16/17 complex. This region was observed in the framework of an extended cluster search. I used near infrared observations to look for the missing cluster that are yet to be found in our Galaxy in order to unveil the mysteries of star formation in the inner parts of the Milky Way.
Alexander Karim
Star Formation in the COSMOS Field : A radio view on the build-up of stellar mass over 12 billion years (thesis pdf, 15MB) Supervisors:
Eva Schinnerer / Hans-Walter Rix (MPIA)
Thesis abstract: In this thesis I study the evolution of galaxies with a special focus on their star forming ability by using the extensive multi-wavelength data sets available for the 2 square degree COSMOS deep field. The deep radio continuum data from Very Large Array observations at a frequency of 1.4 GHz (a wavelength of 20 cm) form the basis of my analysis of the cosmic star formation history unaffected by dust obscuration.
A newly developed stacking algorithm enabled an unprecedentedly representative view on the evolution of the average star formation rate within galaxies down to low limiting (stellar) masses since a redshift of z~3 (i.e. ~2 billion years after the Big Bang). My findings are in good agreement with results from different star formation diagnostics that often suffer from large dust corrections or significantly worse statistics. A main result of this thesis is the identification of a constant characteristic mass for star forming galaxies. It implies that galaxies with masses similar to our Milky Way have always been the main sites of star formation. Therefore the often debated 'downsizing scenario' where the characteristic mass decreases with cosmic time is ruled out. In the young universe (<1.5 billion years of cosmic age) I successfully searched for the most extreme star forming environments. These provide critical constraints on cosmic structure formation and dust enshrouded star formation at early times. A detailed case study not only reveals large amounts of molecular gas but also a powerful hidden active galactic nucleus in one such massive starburst. This finding demonstrates the diversity of this cosmologically important galaxy population.
Mauricio Cisternas
Galaxies and supermassive black holes evolving in a secular universe (thesis pdf, 2MB) Supervisors:
Knud Jahnke (MPIA)
Thesis abstract: Most massive galaxies host a supermassive black hole (BH), that had most of its mass built up throughout bright periods of vigorous accretion, during which it is referred to as an active galactic nucleus (AGN). In the local universe it has been observed that BH mass (M_BH) follows tight correlations with various properties of the galactic bulge in which it resides. This led to the currently popular ``co-evolution'' picture, in which most present-day galaxies went through at least one active phase in the past, during which a link between galaxy and BH gets established. However, more robust observational constraints are required on how galaxy and BH related at earlier times, and which mechanism is responsible for triggering these BH growth phases. This thesis studies a large sample of AGN out to z~1 from the COSMOS survey, selected from their X-ray emission and imaged in finest detail with the Hubble Space Telescope, allowing the study of growing BHs together with their host galaxies. We present new constraints on the ratio of BH mass to $total$ galaxy stellar mass (M_*) over the last 7 Gyr for 32 type-1 AGN. We show that the M_BH-M_* ratio at z~0.7 is consistent with the local relation between BH mass and galactic bulge mass. For these galaxies to obey the local relation only a redistribution of disk-to-bulge mass is needed, likely driven by passive secular evolution.
We then tackle and answer a 30-year old question: what is the relevance of major mergers and interactions as triggering mechanisms for AGN activity? We visually analyze the morphologies of 140 AGN out to z~1 looking for signatures of recent mergers, and compare them with a control sample of over 1200 matched inactive galaxies. We find that the merger fraction of the AGN host galaxies is statistically identical to the corresponding inactive galaxy population, at roughly 15%. Together with the fact that the majority of the AGN host galaxies are disk-dominated, unlikely relics of a recent major merger, these results are the strongest evidence to date that secular evolution rather than major merging has dominated BH fueling at least since z~1, and that BHs and galaxies have, for the last 7 Gyr, evolved in a secular universe.
Roman Follert
The atmospheric piston simulator for LINC-NIRVANA and Interferometric observations of massive young stellar objects (thesis pdf, 15MB) Supervisors:
Tom Herbst / Hans-Walter Rix (MPIA)
Thesis abstract: The scope of this dissertation is two - fold. The first part describes the design, construction and verification of the atmospheric piston simulator. The atmospheric piston simulator is a device which will allow the verification of the LINC-NIRVANA fringe tracking system under as realistic as possible conditions. The design of the atmospheric fringe tracker is based on optical fibers for light guiding and integrated optics components for the manipulation of the phase of the transmitted light. This fiber based setup allows on the one hand an easy an reliable way to introduce differential piston sequences, on the other hand integration into LINC-NIRVANA will be trivial. The second part of this dissertation describes the application of Interferometry in the case of the observation of massive young stellar objects. Two objects are presented and discussed in detail: the Kleinmann & Wright object and AFGL 2136. The interferometric measurements as well as the data reduction are described. The spectrally dispersed visibility curves and the differential phase data are fitted by analytical geometrical models. Then, the interferometric data and results from a spectral energy distribution analysis are combined and compared to Monte-Carlo radiative transfer simulations. The results of this analysis indicate the existence of circumstellar disks similar to those found in low mass star formation. A detailed analysis of the mid-infrared silicate absorption features also indicates significant dust grain evolution in AFGL 2136.
Oliver Porth
Formation of Relativistic Jets : Magnetohydrodynamics and Synchrotron Radiation (thesis pdf, 20MB) Supervisor:
Christian Fendt (MPIA)
Thesis abstract: In this thesis, the formation of relativistic jets is investigated by means of special relativistic magnetohydrodynamic simulations and synchrotron radiative transfer. Our results show that the magnetohydrodynamic jet self-collimation paradigm can also be applied to the relativistic case. In the first part, jets launched from rotating hot accretion disk coronae are explored, leading to well collimated, but only mildly relativistic flows. Beyond the light-cylinder, the electric charge separation force balances the classical trans-field Lorentz force almost entirely, resulting in a decreased efficiency of acceleration and collimation in comparison to non-relativistic disk winds. In the second part, we examine Poynting dominated flows of various electric current distributions. By following the outflow for over 3000 Schwarzschild radii, highly relativistic jets of Lorentz factor 8 and half-opening angles below 1 degree are obtained, providing dynamical models for the parsec scale jets of active galactic nuclei. Applying the magnetohydrodynamic structure of the quasi-stationary simulation models, we solve the relativistically beamed synchrotron radiation transport. This yields synthetic radiation maps and polarization patterns that can be used to confront high resolution radio and (sub-) mm observations of nearby active galactic nuclei. Relativistic motion together with the helical magnetic fields of the jet formation site imprint a clear signature on the observed polarization and Faraday rotation. In particular, asymmetries in the polarization direction across the jet can disclose the handedness of the magnetic helix and thus the spin direction of the central engine. Finally, we show first results from fully three-dimensional, high resolution adaptive mesh refinement simulations of jet formation from a rotating magnetosphere and examine the jet stability. Relativistic field-line rotation leads to an electric charge separation force that opposes the magnetic Lorentz force, such that we obtain an increased stability of relativistic flows. Accordingly, the non-axisymmetric modes applied to the field-line foot-points saturate quickly, with no signs of enhanced dissipation or disruption near the jet launching site.
Bhargav Vaidya
Theory of disks and outflows around massive young stellar objects (thesis pdf, 19MB) Supervisors:
Christian Fendt / Henrik Beuther (MPIA)
Thesis abstract: The inner most regions around massive young stellar objects (YSO) are associated with complex interactions between numerous physical processes. Since the inner few Astronomical Units (AU) are tough to resolve observationally, a theoretical approach is important to create a qualitative picture for these regions around young high-mass stars. This thesis investigates the interplay between important physical processes with respect to the dynamics of jets and inner accretion disks. This thesis provides a bridge between the physical structures of the inner and the outer disk, where the later is observationally easier to access. Above all, the importance of the radiative force in altering the dynamics of a magnetically launched jet is outlined in this thesis.
A thin accretion disk model with proper gas and dust opacities is applied for a luminous young high-mass star. This study has furnished estimates of various physical quantities in the inner few AU of the accretion disk. In particular, I have found that the mid-plane temperature around 0.1 AU could be as high as 10^5 K for a 10 Msun star. Such high temperatures in the disk destroy most of the dust grains already at large radii from the central star. This in turn reduces the opacity of the accreted matter thereby overcoming the central radiation-pressure from the young massive star. In addition, such disks are stable to gravitational fragmentation inside of 100 AU from the central star. Thus they form an ideal launching base for long-lasting outflows.
Outflows and jets are an ubiquitous phenomenon in young massive star forming regions. Observational surveys have suggested that the outflows become wider as the star grows in luminosity (thus mass) with time. I have performed magneto-hydrodynamical simulations of jet launching in presence of radiative forces from the luminous star and the inner hot accretion disk. The major outcome of this work, is that the radiative force from the central star plays a dominating role in accelerating and de-collimating the magnetically launched jet, while the influence of the disk radiative force is rather small. In addition, conducting an extensive parameter study, I have found that the outflows become wider as the mass (or luminosity) of the central star increases. The degree of collimation is also affected by the magnetic field strength and optical thickness of the line. This interplay of radiative and magnetic forces provides a physical insight to the trend in degree of collimation suggested by observations.
Finally, a fully three-dimensional simulation is conducted to understand the manner in which the inner accretion disk transports material onto the central massive star. The hydrodynamic flow in the disk is simulated in the presence of radiative transfer and/or self-gravity. The transport of angular momentum is solely due to gravitational torques. My first results indicate that a locally isothermal disk becomes gravitationally instable and fragments in the inner parts as it is fed with matter from the outer massive core with a steady accretion of 10^-3 Msun yr^-1. About 10% of the mass added onto the disk is accreted onto the central star in form of clumps. On the other hand, no fragmentation is seen in an adiabatic disk whose initial temperature profile is consistently derived from radiative transfer calculations. This investigation complements the above semi-analytical study of the inner disk to single out the physics of angular momentum transport in massive accretion disks.
Rene Andrae
Alignment of disc galaxies : Testing the theory of angular-momentum acquisition (thesis pdf, 7MB) Supervisors:
Knud Jahnke (MPIA)
Thesis abstract: This thesis is concerned with the formation of disc galaxies, a special type of galaxies characterised by ordered rotation. The main focus is on testing predictions of theories that try to explain the angular-momentum acquisition of these galaxies. A major issue concerns the proper quantification of galaxy morphologies. This is of vital importance for automated and reproducible classification of disc galaxies in large data samples. The most promising approach is to expand galaxy morphologies into orthonormal basis functions based on a well motived light profile (the Sersic profifile). In the main part of this thesis, the angular momenta of several thousand disc galaxies from the Sloan Digital Sky Survey are analysed. It is demonstrated that previous investigations lacked a rigorous account for the relevant error sources, which lead to unjustified confirmations of theoretical predictions. If all relevant errors are considered, there is no statistical evidence confirming these predictions. The methods developed here are also relevant in a wider context, as they are applicable to other astrophysical investigations. This thesis concludes with a detailed discussion of potential improvements of measurements and an outline of an observational strategy that will allow for a more decisive empirical test of the theoretical predictions.
Ekaterina Lüttjohann
Chemische und mineralogische Entwicklung Protoplanetarer Scheiben (thesis pdf, 4MB) Supervisor:
Hans-Peter Gail (ITA)
Thesis abstract: Modern measurement techniques provide a lot of information about the chemical composition of comets and protoplanetary disks. Studies of the istotopic composition of meteorites provide further information about the conditions of formation for these compounds. However, the history of formation and the evolution paths of these species are often not known.
This work shows that the oldest compounds of the solar system, the calcium aluminum rich inclusions (CAIs) may form during the collaps of a molecular cloud. After the formation they are transported to distances several ten AU away from the region of their formation.
Furthermore, the destruction mechanisms of presolar SiC particles are studied in this work. The calculations show that the ratio between presolar SiC grains and presolar carbon grains in meteorites cannot be explained solely by oxidation processes in the solar nebula.
Another chapter of this work describes the chemical evolution of hydrocarbons in protoplanetary disks. The gas phase chemistry is calculated using methods of chemical kinetics. For this purpose 45 species and 220 reactions are included in the calculation. It is shown that the destruction of carbon grains leads to the formation of hydrocarbons in the inner parts of protoplanetary disks.
Christine Ruhland
Signposts of Hierarchical Merging (thesis pdf, 22MB) Supervisors:
Eric Bell / Hans-Walter Rix (MPIA)
Thesis abstract: In a Lambda-CDM universe, the merging of galaxies plays a key role in their formation and evolution. In this thesis I focus on two aspects of this picture.
The first one is the accretion of satellite galaxies onto disk galaxies, specifically the Milky Way, leaving the galaxy mostly undisturbed in its morphology, but contributing to the stellar populations of its faintest, but most extended component, the stellar halo. I have developed techniques for detecting and exploring stellar streams formed through the tidal disruption of satellites. The main results are the discovery of a candidate stellar stream, and detailed characterization of the Sagittarius tidal stream, a key probe of the Milky Way potential.
In the other project, I explore the evolution of early-type galaxies—thought to be the result of merging of disk galaxies. I show that the observed evolution of their color-magnitude relation (a probe of their stellar populations) is consistent with a constant addition of galaxies with recently truncated star formation. On both scales these studies are consistent with the notion that merging has played an important role in shaping the galaxies as we see them in the present day universe and continues to do so.
Ioanna Arka
Non-linear waves in the laboratory and in astrophysics: Pair production in counter-propagating laser beams and strong waves in pulsar winds (thesis pdf, 5MB) Supervisor:
John Kirk (MPIK)
Thesis abstract: In this work we are investigating non-linear electromagnetic waves in two different physical environments: laboratories on earth and the astrophysical objects known as pulsars. In the first part of our work the interaction of electrons and positrons with strong waves in the form of high intensity laser beams is analyzed. The possibility of emission of energetic radiation which can result in prolific pair production in the focus of two short, counter-propagating ultrahigh intensity laser pulses is examined, taking into account several different possibilities for the relative polarizations and the waveform of the beams. The conclusion is reached that in the next generation laser facilities currently under construction mainly in Europe, like ELI and the 10PW Vulcan laser, pair production and electromagnetic pair cascades should be observed for intensities as low as 1024Wcm−2.
In the second part of this work we focus on large amplitude, low frequency waves that are emitted by pulsars. After a brief review of the current understanding of pulsar winds and the problems inherent to it, we show that the interaction of a relativistic striped pulsar wind with the the termination shock should result in reflection of electromagnetic energy in the upstream, which can affect the outflow, creating a precursor. We then investigate the possible conversion of the pulsar wind to a superluminal linearly polarized wave propagating upstream of the termination shock and show that this will result in the transfer of energy from the fields to the outflow particles in the precursor, lowering the magnetization of the outflow and opening the way for further particle acceleration at the shock front.
Leonard Burtscher
Mid-infrared interferometry of AGN cores (thesis pdf, 27MB) Supervisor:
Klaus Meisenheimer (MPIA)
Thesis abstract: Active Galactic Nuclei (AGNs) are among the most luminous objects in the universe and are classified into a number of types and subtypes. Unified models of AGNs explain some of this variety as a result of different viewing angles towards their cores instead of intrinsic differences: from some viewing angles, our line of sight would be blocked by a dusty torus.
The spatial resolution necessary to resolve these warm dust structures at 10 micron is currently only provided by the Very Large Telescope Interferometer (VLTI) in Chile. First, extensive observations of the radio galaxy Centaurus A are examined that show an extended structure, which is probably connected to the Northern nuclear radio jet, at a distance of about 40 milli arcseconds (0.7 parsec) in front of the nucleus. Secondly, a study of the brightest so-called type 1 Seyfert galaxy, NGC 4151, reveals for the first time a resolved nuclear emitter in such a source. Its properties (size, temperature profile, emissivity) are similar to those of the alternative type 2 galaxies, studied previ- ously. This is consistent with, though not necessarily confirmation of, unified models. Since the previous studies of type 2 source and modern torus models show a wide variety of nuclear dust structures, finally, the so-far most comprehensive study of resolved nuclear dust emission of AGNs is set up to study 13 AGNs of various luminosities L and distances reaching several hundred Mega Parsec. The first full analysis of this study shows in all but one galaxy that tori can be resolved and that their size on parsec scales is not simply proportional to L^0.5.
Christian Angrick
On the derivation of an X-ray temperature function without reference to mass and the prediction of weak-lensing number counts from the statistics of Gaussian random fields (thesis pdf, 5MB) Supervisor:
Matthias Bartelmann (ITA)
Thesis abstract: We present a novel approach for the derivation of the X-ray temperature function for galaxy clusters, which is based on the statistics of Gaussian random fields applied to the cosmic gravitational potential. It invokes only locally defined quantities so that no reference to the cluster's mass is made. To relate linear and non-linear potential and to take into account only structures that have collapsed, we include either spherical- or ellipsoidal-collapse dynamics and compare both resulting models to temperature functions derived from a numerical simulation. Since deviations from the theoretical prediction are found in the simulation for high redshifts, we develop an analytic model to include the effects of mergers in our formalism. We jointly determine the cosmological parameters Omega_m0 and sigma_8 from two different cluster samples for different temperature definitions and find good agreement with constraints from WMAP5. Introducing theoretically a refined detection definition based on the upcrossing criterion, we reformulate our analytic approach for 2D and use it to predict the number density of spurious detections caused by large-scale structure and shot noise in filtered weak-lensing convergence maps. Agreement with a numerical simulation is found at the expected level.
Sanjaya Paudel
Early-type dwarf galaxies: Insight from stellar population studies (thesis pdf, 10MB) Supervisor:
Thorsten Lisker (ARI)
Thesis abstract: We have studied the stellar population properties of a sample of early-type dwarf galaxies (dEs) in Virgo cluster. We derived the simple stellar population (SSP) parameters age and metallicity using the method of Lick indices. We found that not all dEs exhibit the same stellar population properties. The dEs with disc features are relatively younger and more metal enhanced than dEs without disc. We also found the nuclei of dEs have smaller ages and higher metal content than the respective galactic main bodies. Comparing the SSP parameters at the same local density, where the Ultra Compact Dwarf galaxies (UCDs) are located, we do not find any diㄦence in the stellar population properties of dEs nuclei and UCDs. We confirmed that the metallicity correlate with the galaxies luminosity in all classes of earlytype galaxies i.e. Es, dEs and dSphs and dEs luminosity-metallicity relation has the steepest slope among all classes. We discovered a clear break in this relation in the dEs region. The bright and metal rich dEs seem to follow a faint end extension of Es and the metal poor and old dEs either hold a genuine class of early-type dwarf galaxies or just a bright end extension of dSph.
Anne Bochow
A systematic study of Supernova Remnants as seen with H.E.S.S. (thesis pdf, 2MB) Supervisor:
Werner Hofmann (MPIK)
Thesis abstract: Supernova remnants (SNRs) are the remainders of extremely energetic explosions occurring at the end of a star's life. With the energy released during the supernova explosion they are believed to accelerate charged particles to energies of up to 10^15 eV. In the very-high-energy (VHE, > 10^11 eV) gamma-ray band, SNRs represent one of the most populous classes of Galactic sources. Due to its unprecedented sensitivity, H.E.S.S. was the first instrument to allow for a morphological resolution of individual SNRs, proving the existence of particle acceleration and subsequent VHE gamma-ray emission. However, to date many more SNRs are known in the radio waveband than in VHE gamma-rays. This work presents a systematic study of the VHE gamma-ray signal of a sample of around 200 radio SNRs. The VHE gamma-ray-signal of these SNRs is studied individually. Besides the spatial correlation of radio SNRs and VHE gamma-ray sources, the measured flux of VHE gamma-rays is compared to theoretical flux predictions. These predictions are based on assumptions of the total explosion energy, the particle acceleration effciency, the density of the surrounding medium and the distance of the SNRs. The results presented here suggest that these parameters can vary strongly for individual SNRs. Future observations of SNRs in VHE gamma-rays and other wavebands will help to constrain the parameter space and will allow to further discuss acceleration mechanisms in SNRs.
Nikolay Nikolov
A Photometric Study of Transiting Extrasolar Planets (thesis pdf, 50MB) Supervisors:
Cristina Afonso / Thomas Henning (MPIA)
Thesis abstract: In this thesis I present observational methods to detect and characterize transiting extrasolar planets. I study the most common types of astrophysical false positives, identified by ground based photometric transit surveys. I implement and test three robust methods for the Pan-Planets pipeline to classify false alarms, based on the transit light curve morphology: evaluation of out-of-eclipse ellipsoidal variability; determination of astrophysical parameters from a transit fit; and exoplanet diagnostic (eta).
I obtain results from systematic tests of the pipeline using data from OGLE, Pan-Planets simulations and confirmed transiting exoplanets. I confirm the first method as most reliable in ruling out transit-like events due to stellar binaries. I further, present results from a photometric analysis of data obtained with LAIWO at the 1-m telescope at Wise Observatory. I study the necessary methods to obtain high quality light curves at the millimagnitude level down to R = 16 mag. I perform a search for periodic transit events (1 - 2%) and variable stars and identify 22 transit candidates as well as 20 periodic variables. An analysis of the photometric colors, ellipsoidal variability and the exoplanet diagnostic suggests that between zero and two of the light curves should be produced by exoplanets. Furthermore, I propose a follow-up scheme for their confirmation.
Finally, I obtain high signal-to-noise ratio simultaneous multi-band optical/NIR photometry of WASP-4, during three transits of its exoplanet, with the GROND instrument at the ESO/MPG 2.2-m telescope at La Silla Observatory. I produce and analyze its light curves and measure the planet radius, orbital period, semimajor axis and inclination in conjunction with a study of the transit mid-times. An analysis of WASP-4b transit ephemeris, derived from the results, confirms the lack of transit timing variations (TTVs).
Sofia Lianou
The interacting M81 group of galaxies (thesis pdf, 20MB) Supervisor:
Eva Grebel (ARI)
Thesis abstract: The topic of this Thesis is the role of the environment and interactions on the formation and evolution of dwarf galaxies in nearby groups. The M81 group is a highly interacting group, offering a unique opportunity to study the impact of the environment on shaping the properties of its dwarf galaxy population. We use the Tully-Fisher relation to search for potential tidal dwarf galaxies in the M81 group. No potential tidal dwarfs were identified. Furthermore, we derive photometric metallicity distribution functions and examine the presence of metallicity gradients for nine early-type dwarf galaxies of the M81 group. The comparison of their mean metallicity properties with those of Local Group dwarfs shows that these are similar. Not all of the dwarfs show a metallicity gradient, as is also the case of Local Group dwarfs. The fraction of luminous asymptotic giant branch stars in each dwarf is small, while their fraction as a function of their distance from the M81 galaxy does not show any trend. The results indicate that the studied properties are affected by internal processes. Finally, the method of deriving photometric metallicities is evaluated for dwarf galaxies with complex star formation histories, using Galactic dwarf spheroidals. The resulting mean photometric metallicity properties are in good agreement with spectroscopic measurements, while individual star differences become larger the more complex the star formation is.
Isabel Franco Rico
The origin of low-mass early-type galaxies: A combined view from stellar populations and N-body simulations (thesis pdf, 3MB) Supervisor:
Thorsten Lisker (ARI)
Thesis abstract: The COMBO-17 survey data is used to measure stellar masses of late and early-type galaxies in the redshift range 0 < z < 2.1. Hierachical galaxy formation predicts that early-type galaxies leave the blue cloud and assemble at z~1. The transit from blue cloud to red sequence is yet not fully understood. We explore one formation mechanism that might explain how late-type galaxies can transform in low-mass early type galaxies via galaxy harassment. We aim to understand the formation of early-type dwarf galaxies through the so-called galaxy harassment scenario. We perform N-body simulations in which a three-component galaxy (disk + bulge + halo) enters a galaxy cluster in an eccentric orbit. While the galaxy is falling, it gets tidally disrupted by close encounters with other galaxies that populate the cluster. Such encounters are sufficiently vigorous to morphologically transform the galaxy into an early-type dwarf. Through a combined N-body and stellar population approach, we assign multiple stellar generations to the simulated galaxy and trace their evolution. This allows us to extract observable quantities, in particular integrated colours, in order to compare the simulation results to observed Virgo cluster early-type dwarfs. Based on this analysis we discuss whether harassment is able to form typical early-type dwarf galaxies.
Maximiliano Leonardo Moyano D'Angelo
A search for transiting extrasolar planets with the LAIWO instrument (thesis pdf, 20MB) Supervisor:
Thomas Henning (MPIA)
Thesis abstract: In this thesis we study the necessary methods to perform a transit search for extrasolar planets. We apply these methods to search for planets in one of the fields of the LAIWO project: the Cygnus-Lyra field (Laiwo VI). We describe the problems that systematic effects can introduce for precise relative photometry at the millimagnitude level (~3mmag). Ways to minimize and quantify this correlated noise are also described. We test the weaknesses and strenghts of two transit detection algorithms (TDA) namely the Box fitting algorithm (BLS) and the TRUFAS algorithm using archive data from the OGLE project and simulations of the first year of the Pan-Planets survey. These projects are similar in terms of telescope size and field of view to the LAIWO survey. We have found that the main limitations of the BLS algorithm are the transit depth and correlated noise (Red Noise). The TRUFAS detection efficiency correlates with the number of points in transit and the number of transits present in the light curve, and, its detection efficiency is low (less than ~ 50%) for these type of ground-based observations. Finally, we create from the LAIWO data light curves which are suitable to detect planets among the stars brighter than R = 16.5 mag. We have found 31 eclipsing binaries and 18 light curves that have transits consistent with a planet. Of these detections, 3 eclipsing binaries and 8 planet candidates were independently found by the KEPLER survey. Of the 10 newly discovered transiting planets, 3 are promising to justify follow-up confirmation studies, which are always necessary to probe the planetary nature of a transiting companion.
Hsiang-Hsu Wang
Gas evolution in disk galaxies: disk stability, gas accretion & the impacts of spiral density waves (thesis pdf, 4MB) Supervisors:
Ralf Klessen (ITA), Kees Dullemond (MPIA)
Thesis abstract: This thesis studies many aspects of gas evolution in disk galaxies. A simple, effective method is developed for initializing a three-dimensional gaseous disk which is in detailed equilibrium. With this method, theoretical predictions for disk stability and swing amplification are numerically studied for three-dimensional disks. The missing link between intergalactic gas accretion and the star formation activity is found for the galaxy M83. We improve the analysis method to search for the signature of gas infall. For the first time, gas accretion with sufficient fresh gas to fuel star forming disk is kinematically confirmed. The impacts of spiral density waves on gas motions are studied numerically. Shock driven turbulence is quantified and is found to match excellently with observations. Furthermore, the evolution of shock itself has profound impacts on redistributing gaseous surface density, angular momentum and on the development of substructures.
Min Fang
The disks and accretion behavior of Young Stellar Objects (thesis pdf, 10MB) Supervisors:
Roy van Boekel, Thomas Henning (MPIA)
Thesis abstract: Circumstellar disks play an essential role in the star formation process and are thought to be the sites where new planetary systems come into being. Knowledge of the evolution of these objects is pivotal to our understanding of star and planet formation. In this thesis, I present three studies aimed at characterizing the properties of newly formed stars and their circumstellar accretion disks in three qualitatively different environments: the sparse stellar association epsilon Cha, the L1630 and L1641 regions of clustered and distributed star formation in Orion, and the massive cluster Pismis 24. I combined infrared observations, taken from the ground and with the Spitzer Space Telescope, with optical imaging and spectroscopy obtained with various ground-based facilities. I characterized the disk geometry and evolutionary state using the infrared data, and from the optical data I determined the stellar properties as well as the rate at which disk material is accreted onto the central stars. Two important insights gained through my studies are: (1) the disk lifetimes for stars formed in isolation or sparse stellar associations are longer than those of stars formed in relatively dense clusters, whereas the disk lifetimes in clusters harboring very massive stars with spectral types earlier than ~O5 are shorter than those in otherwise similar clusters that lack such very massive stars; (2) the accretion rates show a steeper dependence on the stellar mass in the sub-solar mass regime than in the solar or intermediate mass regime.
Alumni - 2010
Baybars Külebi
Constraints on the origins of magnetic White Dwarfs (thesis pdf, 15MB) Supervisor:
Stefan Jordan (ARI)
Thesis abstract: The central theme of this work is the most frequent final stage of the evolution of magnetic stars, the Magnetic White Dwarfs (MWDs). Improved statistical investigations coming from new surveys and very precise observations of unique MWDs offer the possibility to test various hypotheses on the evolution of these objects. In the first part of our work we identify hydrogen-rich MWDs (DAHs) in the Sloan Digital Sky Survey (SDSS) and investigate the population statistics of all known DAHs in the SDSS. Additionally, we investigate the evolutionary histories of a few of these objects using constraints from the observations of their binary counterparts or through their membership in open clusters. In the second part of our work, we investigate the unique MWD REJ 0317-853, by a parallax measurement with the Hubble Space Telescope and by time resolved spectro-polarimetric modeling. We show that the assumption of centered magnetic dipoles for the field geometry is not correct for more than half of the objects in our sample; this is in particular true for REJ 0317-853 which shows a very uniform field during one rotation phase. This is validated by the first observation of cyclotron absorption in the polarization spectrum of a white dwarf, which is explained with a new model for the self-consistent physical treatment of this absorption process. Furthermore, we study the possible influence of magnetism on the mass loss during the stellar evolution and on the structure of the star which is of importance to understand the nature of the massive white dwarf REJ 0317-853.
Denija Crnojevic
Resolved stellar populations of dwarf galaxies in the Centaurus A/M83 group (thesis pdf, 47MB) Supervisor:
Eva Grebel (ARI)
Thesis abstract: Dwarf galaxies, fundamental ingredients of galactic structures in our Universe, are ubiquitous and surprisingly heterogeneous systems. The study of such objects in nearby groups is a powerful way of investigating their formation and evolutionary mechanisms. The Centaurus A/M83 group is denser and in a more advanced evolutionary phase than our own Local Group, thus being an appealing target for the study of dwarf galaxies. Its more than 50 dwarf members, with different morphologies and stellar contents, can still be resolved into individual stars (at an average Galactocentric distance of ~4 Mpc). We use optical (HST/ACS) and near-infrared (VLT/ISAAC) photometric data to derive physical properties and evolutionary histories for sixteen dwarfs in this group. Specifically, the predominantly old populations of our target early-type dwarfs show metallicity contents that resemble what is found for Local Group members. However, they intriguingly contain lower intermediate-age population fractions than those found in morphologically similar dwarfs around the Milky Way, thus being more comparable to the M31 companions. We also apply our analysis techniques to the deeper photometric data available for M31 early-type dwarfs. The star formation histories derived for our star-forming late-type dwarf targets reveal that the star formation is quenched for galaxies currently found in the densest group regions. The time-dependent spatial distribution of their stellar populations points toward a stochastic star formation mode in these objects. Our results show that the evolution of dwarf galaxies depends on their mass but that it is, at the same time, heavily influenced by the surrounding environment. The Centaurus A/M83 group, along with other nearby galaxy groups, deserves further attention in order for us to ultimately gain deeper insights into the processes that regulate the evolution of dwarf galaxies.
Massimo Viola
On shear and flexion measurements and properties of dark matter halos (thesis pdf, 7MB) Supervisor:
Matthias Bartelmann (ITA)
Thesis abstract: In this work we analyse the Kaiser-Squires-Broadhurst method (KSB) to estimate gravitational shear from surface-brightness moments of small and noisy galaxy images. We show how KSB relies on restrictive mathematical assumptions concerning the relation between convolved and unconvolved ellipticity, the shape of the telescope’s PSF as well as the relation between ellipticity and shear, neither of which hold in practise. We propose improvements to the original KSB relations and we demonstrate that this extension lowers substantially the biases in the shear measurements. Moreover we discuss the impossibility to weaken the assumption on the PSF shape in the KSB framework. For this reason we develop a novel method for weak-lensing measurements, DEIMOS, which is based on a mathematically exact deconvolution of the moments of the apparent brightness distribution of galaxies from the PSFs. We demonstrate the accuracy and capabilities of this new method with a set of specialized tests and show its competitive performance on the GREAT08 challenge data. Moreover we present a possible application of shear measurement for studying proper- ties of galaxy clusters based on linear filtering techniques to constrain the inner slope of the density profile of dark-matter halos. We find that under idealised assumptions, the inner slope is constrained to 15% if the halo concentration c is known, and to < 30% if not. If the signals of many halos can be stacked, their density profiles should thus be well constrained by the proposed linear filters with the advantage, in contrast with strong lensing analysis, to be insensitive to the cluster substructures.
Nicola Da Rio
An Optical Study of Young Stellar Clusters (thesis pdf, 11MB) Supervisors:
Dimitrios Gouliermis, Thomas Henning (MPIA)
Thesis abstract: In the first part of this thesis a new analysis of the Orion Nebula Cluster, one of the most studied star-forming regions in the Galaxy, is presented. Based on multi-band optical photometry and spectroscopy obtained with the Wide-Field Imager (WFI) at the ESO/MPI 2.2-m telescope at La Silla Observatory, I study the systematic effects that bias the derivation of stellar parameters of pre-main sequence (PMS) stars. I derive the new H-R diagram of the entire region, and assign masses and ages to all the members. The age is found to be ∼ 2 − 3 Myr, older than previously estimated. I also confirm the presence of an age spread, and show how the previously found mass-age correlation can be affected by the sample incompleteness and uncertainties in the evolutionary models. In the second part of this thesis, I carry out a research on the low-mass stellar population of the young stellar cluster LH 95 in the Large Magellanic Cloud, based on deep optical photometry with the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope; the deepest ever obtained toward this galaxy, down to V ≃ 28 mag. I isolate the PMS cluster population, and derive the first extragalactic Initial Mass Function (IMF) down to the subsolar regime. It shows a flattening below 1 M⊙, in agreement with the Galactic IMF once results are corrected for unresolved binarity. I study the age distribution of LH 95, introducing a statistical method to derive average age and age-spreads accounting simultaneously for unresolved binarity, differential extinction, variability, accretion and crowding of PMS stars. The best-fit solution for LH 95 suggests an age of ∼ 4 Myr with a gaussian age spread of σ ∼ 1.2 Myr. Finally, I study the early-type highmass stellar population of the cluster, through ground based spectroscopy obtained with the Fiber-fed Extended Range Optical Spectrograph (FEROS) at the ESO/MPI 2.2-m telescope at La Silla Observatory, and photometry from the 1-m telescope at Siding Spring Observatory. The derived stellar masses are combined with my results on the low-mass IMF of the cluster for the study of the most complete extragalactic IMF ever performed.
Olga Zacharopoulou
On the origin of the unusually hard gamma-ray spectra of TeV blazars (thesis pdf, 2MB) Supervisor:
Felix Aharonian (MPIK)
Thesis abstract: The observed VHE spectra from blazars may be significantly modified due to interactions of $gamma$-rays with intergalactic radiation fields. To study the emission production in these kind of objects, one should reconstruct the intrinsic spectra using an Extragalactic Background Light (EBL) model. Interestingly, this correction often leads to unusually hard spectra. In this dissertation we take into account the EBL absorption by using different models to reconstruct blazars' spectra and we study the formation of broad-band spectra in the framework of a proton synchrotron scenario with non-negligible gamma-gamma absorption in the production region. This internal absorption leads to rather hard VHE spectra. Moreover, a significant fraction of the energy absorbed in the VHE band may be transferred into secondary electron-positron pairs providing an additional radiation channel that explains the observed radiation in the Optical/X-ray regimes. In order to demonstrate the potential of the model, we model two relatively distant blazars, 1ES 0229+200 (z=0.1396) and 3C 66A (z=0.444). In addition, we perform numerical simulations using relativistic MHD in time variable injection setup, in an attempt to understand better the conditions under which radiation is produced in relativistic jets.
Tatiana Vasyunina
Very early stages of massive stars (thesis pdf, 27MB) Supervisor:
Thomas Henning (MPIA)
Thesis abstract: The goal of this thesis work is to investigate the initial conditions and the early phases of the formation of high{mass stars by means of dedicated observational studies. We consider two object classes: infrared dark clouds (IRDCs), and a subsequent stage characterized by the presence of young embedded (proto-)stellar clusters. First, we estimate masses and column densities for a completely new sample of southern IRDCs, utilizing our own millimeter continuum emission maps as well as Spitzer satellite data. The parameters we derive show that IRDCs from our sample have the potential to form not only low- and intermediate-mass stars, but can also be the birth places of massive stars. In particular, the comparison of our results with previously obtained data for low-mass starless cores shows a clear trend for IRDCs to have systematically higher column densities. The interpolated IRDC peak column densities exceed the threshold for the onset of massive star formation previously advocated by theoretical studies. We also critically evaluate the limitations of the applied emission and extinction methods. Second, we investigate the physical conditions and the chemical composition of the dense gas within the IRDCs of our sample by means of a multi{line molecular spectroscopy analysis. All the clouds have complex HCO+ line proles, and we detect SiO emission in some clouds. These findings mark the presence of infall and out ow motions in at least some parts of the IRDCs, and hence, ongoing star formation. In comparison with low-mass starless cores, the IRDCs have broader and more intense lines. Thus, they may be characterised by more turbulent conditions. Nevertheless, we find a tendency for the IRDCs to have molecular abundances similar to the case of low-mass starless cores. This indicates similar chemical initial conditions for low- and high-mass star-forming regions. Finally, we study the stellar composition and environmental conditions in the young embedded cluster IRAS 06058+2138. We perform an analysis of all the available data, covering the near- and mid-infrared as well as the centimeter and millimeter wavelength range. In particular new VLT/SINFONI integral{field spectroscopy data have played a pivotal role. We identify three star-forming clusters at diㄦent evolutionary stages. Comparing the relative ages of these clusters and their relative locations, we conclude that triggering could not have led to the cluster configuration we see in the region. Thus, we assume a merely sequential star-formation scenario for this region. With this study, we also demonstrate the importance of a multi{wavelength approach. Combining data at diㄦent wavelengths allows us to investigate both the more evolved stellar population, as well as very young embedded objects, and hence, to reconstruct the star-forming history of structured cluster environments.
Jean-Claude Waizmann
On finding galaxy clusters with Planck and the spherical collapse model in different Dark Energy cosmologies (thesis pdf, 20MB) Supervisor:
Matthias Bartelmann (ITA)
Thesis abstract: One of the main objectives of the Planck mission is to perform a full-sky cluster survey based on the Sunyaev-Zel'dovich (SZ) effect, which leads to the question of how such a survey would be affected by cosmological models with a different history of structure formation than LCDM. To answer this question, I developed a fast semi-analytic approach for simulating full-sky maps of the Compton-y parameter, ready to be fed into a realistic simulation pipeline. I also implemented a filter and detection pipeline based on spherical multi-frequency matched filters, that was used to study the expected SZ cluster sample of Planck. It turned out that realistic samples will comprise ~1000 clusters at low rate of contamination, significantly lower than originally anticipated. Driven by wrong estimates of the impact of early dark energy models on structure formation, we studied the spherical collapse model in dark energy model, finding that models with varying equation-of-state have a negligible impact on the structure formation. Yet, the different expansion history for the different models can be detected via volume effects, when counting objects in a known volume. Furthermore, it turned out that the different expansion history strongly affects the angular SZ power spectra for the various models, making them an interesting tool to distinguish and constrain alternative cosmologies.
Andras Zsom
Monte Carlo models of dust coagulation (thesis pdf, 40MB) Supervisor:
Kees Dullemond (MPIA)
Thesis abstract: The thesis deals with the first stage of planet formation, namely dust coagulation from micron to millimeter sizes in circumstellar disks. For the first time, we collect and compile the recent laboratory experiments on dust aggregates into a collision model that can be implemented into dust coagulation models. We put this model into a Monte Carlo code that uses representative particles to simulate dust evolution. Simulations are performed using three different disk models in a local box (0D) located at 1 AU distance from the central star. We find that the dust evolution does not follow the previously assumed growth-fragmentation cycle, but growth is halted by bouncing before the fragmentation regime is reached. We call this the bouncing barrier which is an additional obstacle during the already complex formation process of planetesimals. The absence of the growth-fragmentation cycle and the halted growth has two important consequences for planet formation. 1) It is observed that disk atmospheres are dusty throughout their lifetime. Previous models concluded that the small, continuously produced fragments can keep the disk atmospheres dusty. We however show that small fragments are not produced because bouncing prevents fragmentation. 2) As particles do not reach the fragmentation barrier, their sizes are smaller compared to the sizes reached in previous dust models. Forming planetesimals from such tiny aggregates is a challenging task. We decided to investigate point 1) in more detail. A vertical column of a disk (1D) is modeled including the sedimentation of the particles. We find that already intermediate levels of turbulence can prevent particles settling to the midplane. We also find that, due to bouncing, the particle size distribution is narrow and homogenous as a function of height in the disk. This finding has important implications for observations. If it is reasonable to assume that the turbulence is constant as a function of height, the particles observed at the disk atmospheres have the same properties as the ones at the midplane.
Ronny Zhao-Geisler
The surprising mid-IR appearance of the Asymptotic Giant Branch stars R Aql, R Aqr, R Hya, V Hya and W Hya: molecular and dust shell diameters and their pulsation dependence probed with the MIDI interferometer (thesis pdf, 20MB) Supervisor:
Andreas Quirrenbach (LSW)
Thesis abstract: Asymptotic Giant Branch (AGB) stars are the main distributors of dust into the interstellar medium due to their high mass loss rates in combination with an effective dust condensation. It is therefore important to understand the dust formation process and sequence in their extended atmosphere. The wind of these stars is driven by strong stellar pulsation in combination with radiation pressure on dust. High-resolution mid-IR interferometry is sensitive to the structure of the stellar atmosphere, consisting of the continuum photosphere and overlying molecular layers, as well as to the properties of the dust shell.
This work studies the location of molecular layers and newly formed dust as a function of pulsation cycle and chemistry, as well as tries to identify molecules and dust species which cause the diameter of the star to vary across the N-band spectral domain (8-13 microns). Mid-IR interferometric data of the oxygen-rich AGB stars R Aql, R Aqr, R Hya and W Hya, and the carbon rich AGB star V Hya were obtained with MIDI/VLTI between April 2007 and September 2009, covering several pulsation cycles. The spectrally dispersed visibility data are modeled by fitting a fully limb-darkened disk in order to analyze the molecular layers, and by fitting a Gaussian in order to constrain the extension of the dust shell. Because uv-coverage was sufficient for R Hya and W Hya, asymmetries could be studied with an elliptical fully limb-darkened disk.
The angular diameters of all Oxygen-rich stars in the sample appear to be about two times larger in the mid-IR than their photospheric diameters estimated from the near-IR. The overall larger diameter in the mid-IR originates from a warm optically thick molecular layer of H2O, and a detected gradual increase longword of 10 microns can be attributed to the contribution of a spatially resolved, optically thin, close corundum (Al2O3) dust shell. A significant contribution of SiO shortward of 10 microns cannot be ruled out for R Aqr. The circumstellar silicate dust shells of all oxygen-rich stars are found to be very extended except for R Aqr. For the carbon-rich star V Hya, it can only be concluded that amorphous carbon and SiC dust is already present close to the star.
The observed angular diameters are smaller at visual minimum than at visual maximum with peak-to-peak variations on the order of 20% to 30% except for W Hya for which the variation is 6%. The diameter periodicity can be explained with the phase-dependent water vapor and corundum dust presence and its temperature sensitivity. Since this variation traces only the location of constituents which are not relevant for the wind formation, no firm conclusions can be drawn concerning the mass loss mechanism. One can only speculate that more dust forms at visual minimum. Cycle-to-cycle variations of the layer traced with MIDI are lower than intracycle variations, and are on the order of 6%. R Hya does not show any deviations from circular symmetry, while an asymmetry of the extended atmosphere of W Hya can be confirmed.
These observations of a larger sample of stars than available before confirm previous results, and emphasize the need for dynamic stellar model atmospheres with consistently included dust formation close to the star. It can also be concluded that interferometric observations in the N-band are an irreplaceable tool to resolve close stellar structures and to search for atmospheric constituents.
Benjamin Moster
Hydrodynamic Simulations of Cosmological Galaxy Merger Trees (thesis pdf, 6MB) Supervisor:
Hans-Walter Rix (MPIA)
Thesis abstract: In this thesis we use cosmological merger trees and semi-analytic models of galaxy formation to provide the initial conditions for multi-merger hydrodynamic simulations. In this way we exploit the advantages of merger simulations (high resolution and detailed treatment of the gas physics) and semi-analytic models (cosmological background and low computational cost), and integrate them to create a novel tool. This approach allows us to study the evolution of various galaxy properties with an improved treatment of the gas components, including, for the first time, the hot gaseous halo from which gas cools and accretes onto the central disc. Using a suite of minor merger simulations we find that disc thickening is reduced relative to the collisionless case through the absorption of kinetic impact energy by the gas. In a following series of major merger simulations, we show that adding the hot gas component is a key ingredient in order to reproduce several observed properties of elliptical galaxies, like the abundance of fast rotators. Moreover, the presence of a gaseous halo reduces the starburst efficiency. We then focus on the effects of multiple concurrent mergers, which we found to be cosmologically more common than sequences of isolated binary mergers. For this, we investigate, whether accreted satellite stars can be distinguished kinematically from stars formed 'in situ' in the central galaxy, and find that this is only possible for a fraction of the disrupted satellites. Our simulations to date indicate that the combination of a detailed treatment of gas physics, high resolution, and a cosmological background, brings numerical simulations in better agreement with observations. Overall, the novel tool developed in this thesis will be very useful for pursuing a number of questions pertaining to the transformation of galaxy internal structure by mergers and accretion.
Kelly Foyle
Spiral Arms and their Effects on Secular Evolution and Star Formation in Disk Galaxies (thesis pdf, 20MB) Supervisor:
Hans-Walter Rix (MPIA)
Thesis abstract: We investigate how spiral structure affects the observational properties of disk galaxies both in terms of dynamical secular evolution and of star formation. We derived the first observational estimate of the torque-induced instantaneous angular momentum flow, resulting from non-axisymmetric features in the stellar distribution for a sample of 24 galaxies. The strongest torques were found among barred galaxies. In the inner regions, the average torques are strong enough to redistribute angular momentum on a timescale of ~4 Gyr with an outward angular momentum flow. In examining the role of spiral arms in star formation we found that they do not dominate, even in grand-design spiral galaxies as there is a comparable amount of interarm star formation. Further, we found that the arms show no enhancement in the efficiency of star formation in terms of molecular gas. We searched algorithmically for angular offsets between star formation tracers and found that there was no systematic spatial ordering of these tracers, as would be predictable by a shock triggering model of spiral structure. It seems spiral structure is most likely transient or at least more complex than the simplest models predict. These results point to a spiral structure that plays a lesser role in shaping a galaxy’s observable properties as was previously thought. The strength of gravitational torques depends more strongly on bars than on spiral structure, and spiral arms are not regions of enhanced star formation efficiency. At best they act to reorganize the interstellar medium and concentrate the gas.
Olaf Schulz
The design study of IceCube DeepCore : Characterization and veto studies (thesis pdf, 15MB) Supervisor:
Elisa Resconi (MPIK)
Thesis abstract: In this thesis the design study for the low energy extension to the IceCube neutrino telescope, called DeepCore, is presented. IceCube and DeepCore are currently under construction at the geographic South Pole, Antarctica. With DeepCore the energy threshold of IceCube is significantly lowered, opening new possibilities to, e.g., improve low-energy point source searches, to measure standard vacuum neutrino oscillations and to increase the sensitivity to solar dark matter annihilation signals. In parallel to the DeepCore design study a, software based, active atmospheric muon veto technique has been developed, which for the first time, allows IceCube to measure neutrinos with a full sky (4 Pi) acceptance, although at a significantly reduced effective volume. Furthermore, in the course of the active atmospheric muon veto development, a new opportunity has been discovered to also reduce the measured rate of atmospheric muon neutrinos, a previously irreducible background to any search for extra-terrestrial neutrinos. This effect has been analytically studied, independent of any detector design. As of today, the first data-taking period with full atmospheric muon veto capabilities lies ahead. This thesis concludes with a first look on early data taken with DeepCore and a first estimation of a sensitivity to a southern hemisphere point source.
Christian Schwab
A calibration system for a Rayleigh laser guide star constellation & a planet around a giant star (thesis pdf, 4MB) Supervisor:
Andreas Quirrenbach
Thesis abstract: My thesis consists of two parts. In Part I, I describe my contributions to the laser guide star adaptive optics facility at the Large Binocular Telescope, the Advanced Rayleigh guided Ground layer adaptive Optics System , ARGOS. I investigate the effect of scattered light from propagating the laser light across the aperture, and find that contamination is not a concern for the science instruments. I present a study of reflective laser launch telescopes based on commercial optics, and compare this to the refractive design chosen as baseline for ARGOS. The discussed options present an interesting alternative with only small additional light loss at substantially reduced cost and procurement risk. I develop a calibration scheme for the full adaptive optics system, based on artificial light sources that illuminate the deformable mirror, imitating the laser guide star beacons. This enables the interaction between deformable mirror and wavefront sensor to be calibrated at any time, greatly enhancing the possibilities and time available for engineering on the installed system. The light source has to be placed in the prime fcous, as ARGOS uses the adaptive secondary mirrors of LBT. The optical design of the calibration light source is not trivial as the guide star constellation comprises of three beacons forming an equilateral triangle on a circle with a radius of 2 arcminutes. The images of the beacons that have to be reproduced suffer from strong aberrations caused by the large off-axis distance. To match the wavefront of the beacons' images to the desired precision of 50 nanometres rms, I designed a custom objective, incorporating a computer generated hologram to shape the wavefronts of three optical fibres forming the light sources. The elliptical front surface of the objective is used in reflection to generate a central, diffraction limited spot serving as an alignment aid and truth sensor for the measurements. A thorough tolerance analysis including the assembly and the alignment at the telescope ensures that the design specifications can be met during operation. The second part of the thesis concerns the search for planets around a sample of nearby giant stars with the Doppler technique carried out at Lick Observatory's CAT telescope. I analyse new data from the previous three years, which together with the existing data from our survey form a database covering 11 years. The radial velocity measurements reveal the presence of a planetary companion around one of our target stars, which was previously unknown. This is the lightest planet found around a giant star to date with a minimum mass of 1.92 Jupiter masses in a 785 day orbit.
Peter Melchior
Shapelets for gravitational lensing and galaxy morphology studies (thesis pdf, 3MB) Supervisor:
Matthias Bartelmann
Thesis abstract: The presented work is concerned with the morphological description of stars and galaxies in the framework of the shapelet method. This method constitutes a linear expansion in the orthonormal set of Gauss-Hermite polynomials. Its main advantages – linearity, compactness, invariance under Fourier transformation, and the relation to the moments of the brightness distribution – are extensively discussed. The practical treatment of the image decomposition and of the deconvolution from the point spread function are further elaborated. Moreover, three fields of application are presented together with new investigations on the applicability and validity of the method: weak gravitational lensing, morphological class discovery, and realistic simulation of extragalactic observations.
Giovanni Natale
Dust emission and star formation in the Stephan's Quintet compact group of galaxies (thesis pdf, 5MB) Supervisor:
Richard Tuffs (MPIK)
Thesis abstract: Understanding the intergalactic medium (IGM) gas cooling processes, which are necessary to fuel star formation in galaxies, and the effect of galaxy-galaxy and galaxy-IGM interactions, which modify stellar and gas distributions of galaxies in groups and clusters, is vital to construct realistic models of galaxy formation and evolution.
The Stephan's Quintet (SQ) compact group of galaxies is a natural laboratory for studying these phenomena because the galaxies of this group are heavily interacting between each other and with the group IGM. Furthermore its vicinity allows to study the details of the interaction phenomena, which are believed to be much more common in the early universe, and its compactness on the sky permits studies of diffuse components associated with the group IGM.
In this thesis we present an analysis of a comprehensive set of MIR/FIR observations of Stephan's Quintet, taken with the Spitzer Space Observatory. The emission seen at these wavelengths is produced by dust particles and can be used to trace star formation events, AGN activity and also hot gas cooling, in the case dust emission is powered by collisions between plasma particles and dust.
Applying a novel fitting technique to the Spitzer FIR maps, we have been able to separate the different sources of dust emission in this group and perform their photometry at FIR as well as MIR wavelengths. Our study has revealed for the first time the presence of a luminous and extended component of infrared dust emission, not connected with the mainbodies of the group galaxies, and roughly coincident with the X-ray halo of the group. We fitted the inferred dust emission spectral energy distribution of this extended source and the other main infrared emission components of SQ, including the intergalactic shock, to elucidate the mechanisms powering the dust and PAH emission, taking into account dust collisional heating and heating through UV and optical photons.
Combining the fraction of dust luminosity powered by UV photons, as derived from the SED fitting, with the UV luminosity directly observed on the GALEX FUV map of SQ, we estimated the star formation rate (SFR) for each dust emitting source, thus providing a complete picture of star formation in SQ embracing obscured and unobscured components. The total SFR of SQ is $7.5~{rm M_odot/yr}$, similar to the value expected for non interacting galaxies of the same mass of SQ galaxies. However the star formation sites are found mainly at the periphery of the galaxies or in the intergalactic medium, at variance with the usual pattern of star formation in field galaxies which is typically distributed in the central regions or main bodies of galaxies. Despite the unusual location of star formation sites, we have found that, for the brightest sources in SQ, the SFR per unit physical area is similar to that characteristic of disk galaxy star formation regions when compared to the corresponding gas column density on a Kennicutt-Schmidt diagram. We also show that even though the detected extended component of dust emission trace the distributed group star formation, available sources of dust in the group halo can provide enough to produce up to $L_{IR}approx10^{42}~{rm erg/s}$ powered by collisional heating. This amount, several times higher than the X-ray halo luminosity, could provide an important cooling mechanism for the IGM hot gas.
At the end of the thesis we present a theoretical model of a high velocity shock, similar to the one occuring in SQ IGM, taking into account dust cooling and dust destruction. This model shows that, although the efficiency of dust cooling drops quickly because of dust removal by sputtering, the gas cooling time is reduced by a factor of 2-3, compared to the case where only radiative cooling is considered.
Yolanda Sestayo de la Cerra
Search for high energy neutrinos from our galaxy with IceCube (thesis pdf, 5MB) Supervisor:
Elisa Resconi / Konrad Bernloehr (MPIK)
Thesis abstract: High energy neutrino telescopes are expected to play a major role in the discovery of the first unambiguous sources of cosmic-rays. With completion in 2011, the IceCube neutrino detector constitutes the most sensitive instrument to sources of high energy neutrinos. Its performance and discovery potential are usually given in the energy range above a few TeV, in order to achieve the best signal to noise for sources following an E^-2 spectrum without an energy cutoff up to at least 1 PeV. However, given the present understanding and multiwavelength picture of our galaxy, we can expect that galactic sources of high energy neutrinos show significant deviations from the E^-2, no cutoff approximation. The common data analysis are therefore not optimal for such galactic scenarios, requiring exposure times of the order of several years, even a decade, to reach a level of sensitivity at which a possible detection starts to be plausible. The main goal of this thesis is to improve the discovery potential of IceCube to galactic sources of high energy neutrinos, aiming to a better understanding of the high energy processes taking place in our galaxy. In order to fulfill this goal, I follow two lines of action: (1) to increase the detection capabilities of IceCube for neutrinos in the energy range between 100 GeV < E < 1 TeV; and (2) to develop a search method which is able to reduce the minimum detectable flux per point source. The improvement of the IceCube performance at energies below 1 TeV is achieved with the use of the combined detector configuration IceCube 22 strings plus AMANDA (Antarctic Muon And Neutrino Detector Array). The data processing scheme is designed in order to keep as many good low energy events as possible. As a result, this analysis achieved the best sensitivity for sources with neutrino spectra steeper than E^-2 and/or an energy cutoff below 1 PeV. The second goal of this thesis is motivated in order to search efficiently for high energy neutrinos from the Cygnus star forming region of the Galaxy. In order to extend the search beyond a single point source, I developed a method based on two-point analysis to detect, within an extended region, event patterns which might go undetected in conventional point source analysis. The results obtained with this method indicate that the minimum detectable flux per point source is reduced by 26%-59% with respect to standard point source analysis, provided there is more than one point source within the region under study. This method was applied on the Cygnus region of the Galaxy using the data sample obtained with the combined detector IceCube 22 strings plus AMANDA, yielding a significance of 2.3-sigma.
Julian Merten
Gravitational lensing : an advanced method to recover the mass distribution of galaxy clusters (thesis pdf, 20MB) Supervisor:
Matthias Bartelmann (ITA)
Thesis abstract: This work shall be on clusters of galaxies. Those most massive, gravitationally bound objects in the observable Universe represent the high-mass tail of the mass function, rendering them as objects of interest for cosmology. Not only that they allow for the derivation of several cosmological parameters, but they are also ideal cosmic laboratories. Direct comparisons between numerical simulations and observations are particularly appealing in the case of clusters, as we will show. Maybe the most promising method to derive cluster properties from observations is gravitational lensing. Light rays of distant background sources are bent on the way to the observer, due to the high mass concentrations of clusters, and thereby carry important information about the deflector. In this work we develop an advanced, nonparametric method to recover the mass distribution of galaxy clusters by combining weak and strong gravitational lensing. The underlying numerical algorithm makes use of modern concepts of high-performance computing and is fully parallelised. We proof the capabilities of our method, compared to established methods, while reconstructing simulated clusters of galaxies and capitalising realistic lensing scenarios. We close our work with the reconstruction of two well-known, strongly lensing clusters, namely MS2137.3-2353 and CL0024+1654.
Christoph Federrath
The formation of molecular clouds and stars by turbulent compression and collapse (thesis pdf, 13MB) Supervisor:
Ralf Klessen (ITA)
Thesis abstract: The goal of my PhD thesis was to improve our understanding of the role of interstellar turbulence in star formation. In particular, the mechanism of turbulence energy injection, the turbulence forcing, is investigated with hydrodynamical simulations. In a systematic comparison, I study the two limiting cases of turbulence forcing: solenoidal (divergence-free) forcing and compressive (curl-free) forcing. I show that these two cases yield significantly different gas density and velocity statistics. The fractal structure of the gas and the turbulent density probability distribution function (PDF) are explored in detail. I find that compressive forcing yields a three times higher standard deviation of the roughly Gaussian density PDF. I discuss the impact of this result on analytic models of star formation. A detailed comparison with observational data reveals that different observed regions show evidence of different mixtures of compressive and solenoidal forcing, with more compressive forcing occurring primarily in swept-up shells. To follow the gravitational collapse of dense gas in numerical simulations, I implemented accreting sink particles in the adaptive mesh refinement code FLASH. Using sink particles, I show that compressive forcing yields star formation rates more than one order of magnitude higher than solenoidal forcing, consistent with analytic models.
Aday Robaina
Eight Giga years of galaxy mergers (thesis pdf, 3MB) Supervisors:
Hans-Walter Rix / Eric Bell (MPIA)
Thesis abstract: Galaxy interactions are expected to play a crucial role in the build-up of stellar mass in any cold dark matter cosmology. Of particular interest are the mergers between systems of a comparable mass, as they are predicted to be one of the main modes of galaxy growth and have a crucial impact in the shaping of galaxy morphologies and dynamics.
In this thesis I study two key aspects of the role that mergers play in galaxy evolution: a) What is the contribution of major galaxy interactions to the star formation history of the Universe at z < 1?, and b) How important are galaxy interactions for the build-up of the massive end of the red sequence?
To answer the first question I use photometric redshifts, stellar masses and UV star formation rates from COMBO-17, 24 microns star formation rates from Spitzer and galaxy morphologies from two deep Hubble Space Telescope cosmological survey fields to study the enhancement in star formation activity as a function of galaxy separation. I apply robust statistical tools to find galaxies in close pairs, augmented with morphologically-selected very close pairs (unresolved in the ground-based photometry) and merger remnants from the Hubble Space Telescope imaging, finding that, on average, major galaxy interactions between galaxies more massive than 10^10 M_sun at 0.4 < z < 0.8 enhance the star formation activity by a factor of less than 2. I carry out detailed modeling of the methodology using a mock galaxy catalog from the Millenium Simulation, finding that in the regime applicable to this work the recovered enhancement in SF rate is accurate to better than 10%, smaller than the other sources of uncertainty.
Accounting for the fraction of merging and interacting systems, I integrate the enhanced star formation to demonstrate that less than 10% of star formation activity is directly triggered by those interactions. To answer the second question I look for close pairs of galaxies on a sample drawn from the COSMOS and COMBO-17 galaxy surveys to find that the fraction of M_* > 5x10^10M_sun galaxies in close pairs (a proxy for the fraction of objects involved in an interaction) were more common 7 Gyrs ago by a factor ~2. By converting this merger fraction to a merger rate I estimate that 70% of the very massive galaxies (M_*>10^11 M_sun) have undergone a merger since z=1.2. This merger rate is sufficient to explain the observed number density evolution of such massive galaxies in the last 7 Gyrs. Merging plays, therefore, a dominant role in the formation of massive galaxies in the Universe.
Rosalind Skelton
(South Africa)
The Effect of Mergers on Galaxy Formation and Evolution (thesis pdf, 4MB) Supervisor:
Eric Bell (MPIA)
Thesis abstract: This thesis explores the effect of galaxy mergers on the evolution of galaxies over the last 8 billion years using the merger trees from a semi-analytic model (SAM) of galaxy formation. The SAM produces reasonable agreement with the distribution of mass, luminosity and colour at low redshifts, as well as the observed merger fractions. I revisit two apparent contradictions between the standard hierarchical model of galaxy formation and observations of early-type galaxies, using the galaxy merger trees as the basis for further modelling. The observed colour--magnitude relation from the Sloan Digital Sky Survey has a change in slope and smaller scatter at the bright end. A simple toy model shows that dry mergers produce similar characteristics. Contrary to previous claims, the small scatter in the observed CMR thus cannot be used to constrain the amount of dry merging. I incorporate stellar population synthesis modelling into this framework to explore the evolution of early-type galaxies since z=1. There is strong evolution in colour and magnitude if no mergers occur after this time. Dry mergers and the recent addition of younger populations onto the red sequence reduce the evolution, mimicking that of an ancient passively-evolving population. Early-type galaxies can therefore appear to have evolved passively even though significant merging activity continues to recent times.
Cassandra Fallscheer
(United States)
Massive Star Formation : the Role of Disks (thesis pdf) Supervisor:
Henrik Beuther (MPIA)
Thesis abstract: In this thesis, I study three different evolutionary stages of the massive star formation process looking for supporting evidence for an accretion-based formation scenario of massive stars. The first source studied, the Infrared Dark Cloud IRDC 18223-3, is at one of the earliest observable phases of massive star formation. This source is characterized by a cone-shaped molecular outflow component which is used to establish the outflow orientation. A velocity gradient traced by the molecule N2H+ but more convincingly by CH3OH is indicative of a rotating object oriented orthogonally to the outflow direction. This object is on the order of 28,000 AU in size and does not exhibit Keplerian rotation, but may host a disk within. Modeling this velocity gradient shows that a single rotating and infalling entity is capable of reproducing the observations.
Moving to a High Mass Protostellar Object, IRAS 18151-1208, a well-defined outflow orientation is observed as well as an elongation in the 1.3 millimeter dust continuum that is perpendicular to the outflow. This elongation is modeled using a Monte Carlo 3D radiative transfer code. Comparing the modeling results to those of low mass protostars it is deduced that a scaled up version of low-mass star formation provides a plausible description of the observations in this high mass case. In the scaled up version, the density and flaring exponents as well as the relative scale height at one third of the outer radius remain the same as in the low-mass model. The disk mass, outer radius, and central star's mass and luminosity all increase.
The third source studied in this thesis, the hot molecular core IRAS 18507+0121, exhibits the rich chemistry characterizing the hot core phase of massive star formation. The outflow orientation is confirmed and each chemical species is looked at for indication of rotation. Somewhat surprisingly, clear signatures of rotation are not detected and several possible explanations for this are discussed such as insufficient spatial resolution. However, along the lines of what has been observed in IRAS 18151-1208, a slight elongation in the dust continuum perpendicular to the outflow orientation is detected.
Several approaches are explored as a means of studying whether the observable differences in the massive star formation regions are a result of evolution. Taken individually, no indicator is sufficient to definitively determine an age sequence for the three sources. However, taken collectively, the trends seen in these case studies can be attributed to an evolutionary sequence. The results of this thesis are consistent with an accretion based formation mechanism of massive stars and I conclude that the structural changes of the observed disk-like structures from large-scale to more compact may be the result of evolution.
Bagmeet Behera
Effects of EBL extinction on the VHE spectra of blazars (thesis pdf, 15MB) Supervisor:
Stefan Wagner (LSW)
Thesis abstract: Active Galactic Nuclei (AGN) are the most powerful steady sources of electromagnetic radiation in the cosmos. Blazars are AGN with their jets pointing straight at us, giving us the opportunity to probe deep into the core which generates this tremendous luminosity. The Very High Energy (VHE; E>100 GeV) photons from blazars while travelling through the intergalactic medium, interact with the low energy photons (in the UV-IR range) constituting the Extragalactic Background Light (EBL), by producing e-e+ pairs, and get absorbed. More than 25 nearby blazars (z<0.3) have been detected in VHE, but due to EBL extinction it is extremely difficult to detect distant sources. This phenomena gives us the means to measure the EBL level which cannot be directly measured. In this thesis the effects of EBL absorption on the spectra of blazars is studied. Two methods to discover distant VHE blazars were explored. The observations with HESS led to the detection of the blazar PKS 1510-089 at z=0.36, at a 4.8 sigma level. A cross check analysis with a more advanced analysis tool confirmed this detection at a 8.5 sigma level. The GeV spectrum obtained by analyzing Fermi GST data, was adopted as the intrinsic source spectrum. Models for the gamma-ray spectrum including the EBL absorption was fitted to the gamma-ray data and it was shown that all the 4 current EBL models considered here, fit the data well. It is concluded that this is due to the sensitivity limits of the current VHE instruments. Stronger constraints on the EBL would only be possible for sources with much harder spectrum. The outlook for future experiments is discussed.
EBL extinction causes bias in the sample of detected blazars. MonteCarlo simulations were used to generate parent samples for the VHE blazars. These were compared to the true VHE blazar sample and constraints on the parent sample properties were drawn. The lack of a spectral softening with z in the true VHE sample, is found consistent with a parent sample that clearly shows this softening, making it unnecessary to assume any special dependence of the intrinsic spectral index on z. The dependence of the evolution of the EBL on the cosmological model considered, was explored. The uncertainty in cosmology was found to be negligible compared to the uncertainty on the EBL. The dependence of the extinction on the error in the EBL models as well as the effect of neglecting the EBL evolution with z was illustrated.
Alumni - 2009
Rolf Kuiper
Modeling the formation of massive stars (thesis pdf) Supervisors:
Hubert Klahr / Thomas Henning (MPIA)
Thesis abstract: I investigate the radiation pressure problem in the formation of massive stars using a newly developed frequency dependent radiation transport module for three-dimensional hydrodynamics simulations. The nature of the radiative impact depending on the morphology of the stellar environment is examined in one-, two-, and three- dimensional monolithic collapse calculations of massive pre-stellar cores. Contrary to previous research, a highly superior frequency dependent stellar feedback is considered, the vicinity of the forming star is resolved down to 1.27 AU, and the evolution is computed for a factor of ten longer. For the first time a broad survey of the parameter space is possible. The simulations demonstrate the need of including the dust condensation front to compute the radiative feedback correctly. Earlier calculations, which ignore these physics, lead to an artificial truncation of the accretion phase. The most fundamental result is that the formation of a massive accretion disk in slowly rotating cores bypasses the radiative flux through the optically thin atmosphere, enabling steady accretion. A revealed close-by gravitational instability in the disk drives a sufficiently high accretion rate to overcome the residual stellar radiation feedback. This mechanisms allow the star to grow far beyond any limit found in earlier calculations.
Thomas Peters
Ionization Feedback in Massive Star Formation (thesis pdf) Supervisor:
Ralf Klessen (ITA)
Thesis abstract: Understanding the origin of high-mass stars is central to modern astrophysics. We shed light on this problem using novel radiation-hydrodynamic simulations that consistently follow the gravitational collapse of a massive molecular cloud, the subsequent build- up and fragmentation of the accretion disk surrounding the nascent star, and, for the first time, the interaction between its intense UV radiation field and the infalling material. We show that ionization feedback can neither stop protostellar mass growth nor suppress fragmentation. We present a consistent picture of the formation and evolution of H ii regions that explains the observed morphology, time variability, and ages of ultracompact H ii regions, solving the long-standing lifetime problem.
Anton Vasyunin
Chemistry in the ISM and disks on the verge of planet formation (thesis pdf) Supervisors:
Dmitry Semenov / Thomas Henning (MPIA)
Thesis abstract: The general purpose of the thesis work is to improve astrochemical models in the era of ALMA. This era is characterized by the active study of protoplanetary disks and the search for extraterrestrial life.
First, we study how uncertainties in the rate coefficients of chemical reactions affect the abundances and column densities of key molecules in protoplanetary disks. We isolate a group of key species which have column densities that are not very sensitive to the rate uncertainties, making them good potential tracers of physical conditions in disks. We identify about a hundred reactions with the most problematic rate coefficients, which need to be determined more accurately in order to improve the reliability of modern astrochemical models.
Second, we build a realistic astrochemical model using a Monte Carlo approach to all chemical processes, which is the first time this has been done. This allows us to properly take into account the stochastic nature of grain surface chemical reactions, which are of essential importance for the formation of organic molecules -- i.e., the precursors of life as we know it. The recent modified rate approach (MRE) of Garrod et al. (2008) is shown to be the most accurate fast approach of accounting for stochastic effects in astrochemical modeling.
Finally, we apply our model to the study of the chemical composition of an evolving protoplanetary disk with grain growth, in order to reveal chemical tracers of this process. For the first time, a state-of-the-art astrochemical model is coupled with a detailed model of grain growth and sedimentation.
Jamie O'Sullivan
Molecular Cooling and Emissions in Large Scale Simulations of Protostellar Jets (thesis pdf) Supervisor:
Max Camenzind (LSW)
Thesis abstract: The origin of infrared molecular emission associated with Class 0 and Class I protostellar outflows (such as HH211 and HH46/47) is still not fully resolved. One successful model for describing such phenomena is the jet-driven outflow model. It proposes that the emission occurs as a high velocity collimated jet outflow shocks, excites and entrains the molecular ambient matter. Although this scenario does achieve significant success in describing the dynamics and morphology of the outflow, the exact nature of the type of shock causing the emission in such a case - J-type or C-type - is still unclear. Physical conditions in the gas, such the ionisation fraction and magnetic field, are crucial parameters determining the type of shock that will form. However, the immediate region around the class 0 sources producing molecular outflows usually consists of dense, high-extinction gas within a molecular core, impeding observational data regarding these details. Therefore, numerical modelling can play an important role in explaining the observed outflows. We have developed and tested a module, implemented within the PLUTO astrophysical code, to simulate the non-equilibrium molecular chemistry and cooling in a jet outflow which is interacting with its surrounding molecular core gas. Using large scale adaptive mesh magnetohydrodynamical simulations, we predict observationally significant amounts of infrared emissions from J-shock excited molecular gas. We find that the emission can be caused either by direct shocking (''prompt entrainment'') or entrainment and ablation of the ambient gas. We find that the nature of this emission is strongly dependent on absolute and relative densities of the jet and ambient medium, and on the presence of moderate magnetic fields (30 - 120 \muG) in the core. Comparing our results with observations, we confirm that the magnitudes for the emission strength agree with those observed in several sources. Furthermore we demonstrate how the appearance of the emission in different sources depends on the parameters explored here.
Javier Rodon
The fragmentation of massive star-forming Regions (thesis pdf) Supervisor:
Henrik Beuther (MPIA)
Thesis abstract: Since its discovery by E. Salpeter in 1955, the high-mass end of the Initial Mass Function (IMF) has been continuously tested, and its slope has not changed from the value -2.35 originally calculated by Salpeter, the "Salpeter value". Furthermore, it is found that this value is universal. It not only describes the mass distribution of stellar masses in the Milky Way but also in other galaxies.
Stars form individually or in systems within molecular clouds, from local condensations of sizes on the order of ~0.01 pc, the so-called "dense cores". In the case of low-mass star-forming regions, it is found that the Core Mass Function (CMF) resembles the Salpeter IMF. However, in the case of massive star-forming (MSF) regions, the answer is not that clear. The first CMF for a MSF region was derived in 2004 by H. Beuther and P. Schilke for the MSF region IRAS19410+2336. They found that this CMF also resembled the Salpeter IMF. Since then, a few more CMFs for MSF regions have been derived, always with exponents comparable to Salpeter. This suggested that the CMF and the IMF are related in a one-to-one or nearly one-to-one relationship, and that the fragmentation processes within a molecular cloud would set the shape of the IMF at an early evolutionary stage.
Attempting to test that scenario, in this thesis I present and analyze high angular resolution interferometric observations of several MSF regions at millimeter wavelengths, describing their protostellar content and deriving their CMF whenever is possible. We confirm the result of Beuther & Schilke (2004) and obtain a CMF with a power-law slope similar to the Salpeter IMF, however for other MSF regions we obtain a CMF with a power-law slope flatter than Salpeter. This difference suggests that the IMF might not be set at the moment of the fragmentation of the cloud, but insted would be a result of the evolution of the cloud, starting with a flatter mass distribution that becomes steeper at later evolutionary stages. This result is not conclusive yet, and we suggest a series of observations that would be needed to fully test it.
Giovanna Pedaletti
Very high energy emission from passive supermassive black holes (thesis pdf) Supervisor:
Stefan Wagner (LSW)
Thesis abstract: The H.E.S.S. experiment, an array of four Imaging Cherenkov Telescopes, widened the horizon of Very High Energy (VHE) astronomy. Its unprecedented sensitivity is well suited for the study of new classes of expected VHE emitters, such as passive galactic nuclei that are the main focus of the work presented in this thesis. Acceleration of particles up to Ultra High Energies is expected in the magnetosphere of supermassive black holes (SMBH). The radiation losses of these accelerated particles are expected to reach the VHE regime in which H.E.S.S. operates. Predicted fluxes exceed the sensitivity of the array. However, strong photon fields in the surrounding of the acceleration region might absorb the produced radiation. Therefore observations focus on those galactic nuclei that are underluminous at lower photon energies. This work presents data collected by the H.E.S.S. telescopes on the test candidate NGC 1399 and their interpretation. While no detection has been achieved, important constraints can be derived from the obtained upper limits on the maximum energy attainable by the accelerated particles and on the magnetic field strength in the acceleration region. A limit on the magnetic field of B<74 Gauss is given. The limit is model dependent and a scaling of the result with the assumptions is given. This is the tightest empirical constraint to date. Because of the lack of signal from the test candidate, a stacking analysis has been performed on similar sources in three cluster fields. A search for signal from classes of active galactic nuclei has also been made in the same three fields. None of the analyzed samples revealed a significant signal. Also presented are the expectations for the next generation of Cherenkov Telescopes and an outlook on the relativistic effects expected on the VHE emission close to SMBH.
Jonathan Downing
Compact binary populations in Globular Clusters and prospects for gravitational wave detection (thesis pdf, 16 MB) Supervisor:
Rainer Spurzem (ARI)
Thesis abstract: The inspiral and merger of compact binary stars will be major detection events for interferometric gravitational wave observatories. These observatories operate most effectively by comparing their output to template waveforms. In order to make these templates the physical parameters of the source population must be understood. Compact binaries in the galactic field have been investigated using population synthesis models but in dense stellar environments interactions can alter the binary population and may enhance the merger rate.
I study compact binaries in star clusters using a Monte Carlo model for the dynamics. I find that the black hole population interacts strongly, leading to an enhancement in both the number of black hole binaries and the black hole binary merger rate. Due to the high interaction rate the majority of black hole binaries are ejected and thus the mergers occur in the galactic field. I find a promising rate of $1-100$ detections per year for the next generation of ground-based gravitational wave detectors and two possible sources for space-based detectors, both highly eccentric. I conclude that star clusters must be taken into account in order to predict accurate event rates for gravitational wave detectors.
Marcello Cacciato
Galaxy-Dark Matter Connection : from Astrophysics to Cosmology (thesis pdf, 16 MB) Supervisor:
Frank van den Bosch (MPIA)
Thesis abstract: Galaxy-galaxy (g-g) lensing represents an ideal technique to constrain the dark matter distribution on galaxy scales. The required accuracy in the signal can be achieved only by stacking many foreground galaxies and averaging the ellipticity of the resulting background galaxies. Unfortunately, the stacking procedure complicates any astrophysical interpretation. In order to extract information from the composite g-g lensing signal, a reliable model of the way galaxies populate dark matter haloes is required. We use a realistic description of the halo occupation statistics based on the conditional luminosity function. It provides a statistical prescription for the number of galaxies with a given luminosity living in dark matter haloes of a given mass. Being “a priori” constrained by the luminosity dependence of the galaxy clustering, it can be used for predicting the g-g lensing signal without any additional tuning. Our model allows a thorough understanding of the different terms contributing to the signal. We carefully explore the effect of the assumptions entering the model. Our theoretical predictions are in very good agreement with SDSS data. Furthermore, we use the sensitivity of this technique to the underlying cosmological model to study the feasibility of a joint analysis of galaxy clustering and g-g lensing as a novel technique to constrain the values of cosmological parameters such as Omega_m and sigma_8 . We show that uncertainties and systematics in the model do not significantly affect the model predictions. We conclude that a combined analysis of galaxy clustering and g-g lensing can be used as a novel, complementary and competitive technique to constrain cosmological parameters.
Surhud More
Galaxy-Dark Matter Connection : Insights from Satellite Kinematics (thesis pdf, 2MB) Supervisor:
Frank van den Bosch (MPIA)
Thesis abstract: A thorough knowledge of the connection between the mass of dark matter haloes and the properties of their central galaxies is crucial to understand the physics of galaxy formation. The kinematics of satellite galaxies is an excellent technique to measure the dark matter halo masses. However, the kinematics can be measured with high signal-to-noise only by stacking the signal around central galaxies with similar properties, which results in various systematic biases and complicates the interpretation of the signal. This thesis presents an analytical framework that accounts for systematic biases and selection effects and aids in the interpretation of the kinematics of satellite galaxies. A new method is established to obtain the average scaling relations between halo mass and central galaxy properties, and the scatter in these relations simultaneously. After a thorough testing of this method using a realistic mock galaxy catalogue, it is applied to the Sloan Digital Sky Survey to extract the halo mass-luminosity and halo mass-stellar mass relationship of central galaxies and their scatter. Comparisons with other probes of these scaling relations, such as galaxy-galaxy lensing, show good agreement which implies that these scaling relations are well established and supported by various astrophysical probes. Physical insights about these scaling relations, in particular their scatter, gained by the analysis of a semi-analytical model for galaxy formation are also presented. Finally, the inferred scaling relations crucially depend on the transparency of the Universe. By performing a test of the “Etherington relation” between the distances measured by standard rulers and by standard candles, a quantitative measure of the cosmic transparency, which is relatively free from astrophysical assumptions, is obtained.
Thomas Greif
The Formation of the First Galaxies (thesis pdf, 16 MB) Supervisor:
Ralf Klessen (ITA)
Thesis abstract: The primary concern of this thesis is to understand the formation and properties of the first galaxies, as well as the influence of the first stars in terms of radiative, mechanical and chemical feedback. In particular, we elucidate the role of turbulence, ionizing radiation by massive Population III stars, mechanical feedback by highly energetic supernovae, and chemical enrichment. In light of the next generation of ground- and space based telescopes, we derive their observational signature in terms of recombination radiation, bremsstrahlung and 21 cm emission. We find that the cumulative 21 cm signal of the first H II regions will likely be observable by the planned SKA, while the recombination radiation from the first starbursts might be observable by JWST. These probes are essential to test the theoretical framework of the first stars and galaxies and shed some light on this elusive period of cosmic history.
Maria Victoria Rodriguez-Ledesma
Rotation Studies of Young Very Low Mass Stars and Brown Dwarfs (thesis pdf, 2 MB) Supervisor:
Rheinhard Mundt (MPIA)
Thesis abstract: Rotation studies over as large a range of ages and masses as possible are important for constraining the angular momentum evolution of young stellar objects (YSO). Of particular interest are the very low mass (VLM) stars and brown dwarfs (BDs), because of the relatively small number of known rotational periods (Prot) in that mass range. This thesis for the first time provides important information on rotational properties of a large sample of young VLM stars and BDs.
The extensive rotational period study of YSOs in the 1 Myr old Orion Nebula Cluster (ONC) presented here is based on a deep photometric monitoring campaign. As a result, 487 periodic variables (PVs) were found (377 of which are new) with estimated masses between 0.5-0.015 M_odot, 124 of which are BD candidates. This is by far the most extensive and complete rotational period data set for young VLM stars and BDs to date. The dependence of the period distribution on mass and level of rotational modulation was investigated and a comparison with higher mass PVs in the ONC and with the PVs in the approx. 2 Myr old open cluster NGC 2264 was carried out. Substellar objects were found to rotate on average faster than the VLM stars, a trend which was already seen for higher mass stars. In addition, the results presented here suggest a dependence of the Prot on position within the ONC, with the objects located near the cluster center showing on average larger Prot. This result can be explained by a possible age spread in the ONC with a somewhat younger central region.
An interesting correlation between Prot and modulation amplitude was found in which PVs with larger amplitudes rotate on average slower than those with smaller amplitudes, which can in principle be explained by different magnetic field topologies.
In addition, 732 periodic variables in the ONC with known JHK colours were analysed in order to investigate whether rotation periods correlate with the presence of circumstellar accretion disks in particular at very small masses. A highly significant rotation-disk correlation was found among the low and very low mass periodic variables (0.075 < M < 0.4 M_odot), in which objects with NIR excess tend to rotate slower than objects without NIR excess. Interestingly, no correlation was found in the substellar regime. Possible reasons for the absence of such a correlation are discussed in detail.
Dominik Schleicher
The early Universe: probing primordial magnetic fields, Dark Matter models and the first supermassive black holes (thesis pdf, 8 MB) Supervisor:
Ralf Klessen (ITA)
Thesis abstract: The goal of this work is to better understand the universe between recombination and reionization and to outline new possibilities to explore it in more detail. This concerns the stellar population, the physics of the early universe, and the formation of the first supermassive black holes. With the reionization optical depth from WMAP 5, I derive upper limits for the strength of primordial magnetic fields and dark matter annihilation / decay, as well as constraints for stellar population models. Further constraints can be found from the gamma-ray and neutrino background, which rule out s-wave annihilation of light dark matter. It was shown that future 21 cm observations will constrain primordial magnetic fields even further. To improve our understanding of the origin of the first supermassive black holes and their high metallicity, I explore how they can be observed with ALMA and JWST between redshift 5 and 15. For this purpose, I estimated and classified the available observables, and I provide several independent estimates for the expected number of high-redshift black holes. In spite of large model uncertainties, one can expect to find at least a few sources in a solid angle similar to the Hubble-Deep-Field.
Marik Barnabe-Heider
Performance and stability tests of bare high purity germanium detectors in liquid argon for the GERDA experiment (thesis pdf, 30 MB) Supervisor:
Stefan Schönert (MPIK)
Thesis abstract: GERDA will search for neutrinoless double beta decay of 76Ge by using a novel approach of bare germanium detectors in liquid argon (LAr). Enriched germanium detectors from the previous Heidelberg-Moscow and IGEX experiments have been reprocessed and will be deployed in GERDA Phase-I. At the center of this thesis project is the study of the performance of bare germanium detectors in cryogenic liquids. Identical detector performance as in vacuum cryostats (2.2 keV FWHM at 1.3 MeV) was achieved in cryogenic liquids with a new low-mass detector assembly and contacts. One major result is the discovery of a radiation induced leakage current (LC) increase when operating bare detectors with standard passivation layers in LAr. Charge collection and build-up on the passivation layer were identified as the origin of the LC increase. It was found that diodes without passivation do not exhibit this feature. Three month-long stable operation in LAr at 5 pA LC under periodic gamma irradiation demonstrated the suitability of the modified detector design. Based on these results, all Phase-I detectors were reprocessed without passivation layer and subsequently successfully characterized in LAr in the GERDA underground Detector Laboratory. The mass loss during the reprocessing was 300 g out of 17.9 kg and the exposure above ground 5 days. This results in a negligible cosmogenic background increase of 5 x 10^-4 cts/(keV kg y) at 76Ge Q for 60Co and 68Ge.
Matteo Bocchi
Magnetohydrodynamic instabilities of astrophysical jets (thesis pdf, 6 MB) Supervisor:
Max Camenzind (LSW)
Thesis abstract: The remarkable stability of astrophysical jets is not yet fully understood and requires further investigation. In order to study the effects of an antiparallel magnetic field topology on the linear stage and nonlinear evolution of the Kelvin Helmholtz (KH) instability, we performed direct numerical simulations to solve the ideal magnetohydrodynamic equations in a variety of initial configurations. Single shear layers presented growth rates of the instability higher than in the uniform (parallel) case, and a typical oscillation wave vector Ka ≃ 0.4. Vortical motions were observed for Alfv´en Mach numbers Ma > 2. The presence of tearing type magnetic islands, driven by the KH instability, reduced the magnetic field enhancement around the perimeter of the KH vortices proper of the KH instability and, subsequently, reduced the value of the magnetic saturation energy as compared to the uniform field case. The extended domain simulations showed an inverse cascade to bigger scales, more turbulent than in the uniform case. The lower magnetic amplification, due to the islands, moved the threshold for three-dimensional (3D) reorganization to a laminar flow from Ma . 50 (uniform) to Ma . 20 (antiparallel). Two-dimensional (2D) spatial slab-jet simulations showed episodic disruption and revival of the flow due to a magnetic field amplification process, previously believed to be present only in subsonic flows. This result, retrieved also in 3D simulations, is the same for uniform and antiparallel magnetic fields.
Claudia Mignone
Model-independent reconstruction of the expansion rate of the universe through combination of different cosmological probes (thesis pdf, 2 MB) Supervisor:
Matthias Bartelmann (ITA)
Thesis abstract: This work proposes a method to constrain the cosmic expansion rate and the linear growth factor for structure formation from different cosmological measurements, without reference to a specific Friedmann model and its parameters. First, a model-independent reconstruction technique to estimate the expansion rate from luminosity distance data has been developed: it converts the integral relation between the expansion function and the luminosity distance into a Volterra integral equation, which is known to have a unique solution in terms of a Neumann series. Expanding observables such as the luminosity distances to type-Ia supernovae into a series of orthonormal functions, the integral equation can be solved and the cosmic expansion rate recovered within the limits allowed by the accuracy of the data. The performance of the method is demonstrated through application to synthetic data sets of increasing complexity, including a toy model with a sudden transition in the expansion rate. With the additional assumption of local Newtonian dynamics, the growth rate for linear structure formation can be calculated from the estimate of the expansion rate, in the redshift interval over which supernovae are available, and employed in the analysis of cosmic shear data: combined to a traditional, Lambda-CDM analysis of the same data set, this approach allows to tighten the constraints on the matter density parameter, Omega_m, and the normalisation of the power spectrum, sigma_8. Furthermore, the method to reconstruct the expansion rate can be applied to angular-diameter distance data from baryon acoustic oscillation experiments; an optimisation of the orthonormal function set employed in the algorithm has also been performed, by means of a principal component analysis.
Dusan Budjas
Germanium detector studies in the framework of the GERDA experiment (thesis pdf, 26 MB) Supervisor:
Wolfgang Hampel (MPIK)
Thesis abstract: The GERmanium Detector Array (GERDA) is an ultra-low background experiment under construction at Laboratori Nazionali del Gran Sasso. GERDA will search for 76Ge neutrinoless double beta decay with an aim for 100-fold reduction in background compared to predecessor experiments. This ambition necessitates innovative design approaches, strict selection of low-radioactivity materials, and novel techniques for active background suppression. The core feature of GERDA is its array of Germanium detectors for ionizing radiation, which are enriched in 76Ge. Germanium detectors are the central theme of this dissertation. The first part describes the implementation, testing, and optimisation of Monte Carlo simulations of Germanium spectrometers, intensively involved in the selection of low-radioactivity materials. The simulations are essential for evaluations of the gamma ray measurements. The second part concerns the development and validation of an active background suppression technique based on germanium detector signal shape analysis. This was performed for the first time using a BEGe-type detector, which features a small read-out electrode. As a result of this work, BEGe is now one of the two detector technologies included in research and development for the second phase of the GERDA experiment. A suppression of major GERDA backgrounds is demonstrated, with (0.93 ± 0.08)% survival probability for events from 60Co, (21 ± 3)% for 226Ra, and (40 ± 2)% for 228Th. The acceptance of 228Th double escape events, which are analogous to double beta decay, was kept at (89 ± 1)%.
Marie-Helene Nicol
Investigation of the stellar population in galaxies since z=2 by NIR photometry (thesis pdf, 8 MB) Supervisor:
Klaus Meisenheimer (MPIA)
Thesis abstract: This study investigates galaxy evolution in the last 10 Gyr of the age of the Universe. To access the high redshift Universe deep near infrared observations have been carried out in the frame of the COMBO-17+4 (Classifying Objects by Medium-Band Observations) survey. Photometric redshifts, rest-frame SEDs and masses have been obtained for 14286 galaxies down to an observed magnitude H=21.7 mag in 0.2 square degree of the A901-field. The red evolved and the blue star-forming galaxy populations distributions have been followed through cosmic time. An evolving colour-magnitude relation as a function of lookback time has been derived in order to separate the red galaxies from the blue ones. Our results show that the galaxy bimodality persist out to z=1.6.The luminosity and the mass function for the red and the blue galaxy populations has been investigated. Our results show that the characteristic magnitude M* for both galaxy population have considerably dimmed between 2>z>0.2. The massive end of the mass function is dominated by the red galaxies at z<1, whereas above z=1 red evolved and blue star-forming galaxy populations contribute in similar numbers.
Marta Zub
Galactic Aspects of Gravitational Microlensing: High Magnification Events and Stellar Limb-darkening of a Source Star in the Galactic bulge (thesis pdf, 4 MB) Supervisor:
Joachim Wambsganss (ARI)
Thesis abstract: Gravitational microlensing provides a powerful tool to search for extrasolar planets of stars at distances of order of several kpc. The suspicion of a planetary signal in the two high magnification events OGLE 2006-BLG-245 and MOA 2006-BLG-099 led us to perform a detailed modelling and analysis of those two events. Based on the comparison of single-lens and binary-lens models, we demonstrate that the observed light curve deviations are not caused by a planetary companion. Our modelling and analysis of four other high magnification events illustrate the possibility to study detection efficiencies of microlensing data sets to planetary companions. We also present a detailed study of the single-lens OGLE 2004-BLG-482 microlensing event, used to measure the brightness profile of the background lensed star located in the Galactic bulge. We performed data reduction and analysis of well sampled observations of this event obtained by the PLANET, OGLE and MicroFUN collaborations in the I, R and clear filters. We also used a high resolution spectrum obtained with VLT/UVES close to the peak of the light curve to determine the fundamental parameters of the source star, that we find to be a cool red M3 giant with Teff = 3667±150K, log g = 2.1±1.0. We then performed a detailed microlensing modelling of the light curve to measure linear limb-darkening coefficients and to provide new diagnostics of such measurements through microlensing. We compare our results to model-atmosphere predictions based on limb-darkening coefficients for the corresponding stellar parameters. Our limb-darkening measurements agree very well with predictions of the model atmosphere, for both linear limb-darkening laws and alternative limb-darkening profiles based on a principal component analysis of ATLAS stellar atmosphere models.
Timo Anguita
A combined observational and theoretical study of gravitationally lensed quasars (thesis pdf, 4 MB) Supervisor:
Joachim Wambsganss (ARI)
Thesis abstract: In this thesis we study gravitational lensing of quasars, from strong lensing to microlensing. We investigate one strong lens candidate recently discovered in the COSMOS field: COSMOS 5921+0638. Our analysis of the nature of the lens reveals that the system is composed of an early type foreground galaxy at redshift z_l=0.551+/-0.001 lensing a background low luminosity AGN and its host galaxy at a candidate redshift of z_s=3.14+/-0.05. We show that flux anomalies observed in the lensed images are likely due to substructure or microlensing by stars in the lensing galaxy. Extending the analysis of the optical emission of lensed quasars, we have used integral field spectroscopy to study four gravitationally lensed quasar systems: HE 0230-2130, RX J0911+0551, H 1413+117 and B 1359+154, as well as objects in their line-of-sight. The first three systems show anomalous flux ratios consistent with microlensing by stars in their lensing galaxies. In the final part of this work, we probe the structure of the accretion disk of the lensed quasar Q 2237+0305 by the analysis of a high magnification microlensing event seen in the so-called "image C" of the system in the year 1999. Using multi-band observations and microlensing simulations, we measure an accretion disk size of Gaussian width sigma_g'=4.6^{+3.4}_{-3.4} x 10^15 sqrt{M/0.1M_sun} cm and a ratio sigma_r'/sigma_g'=1.3^{+0.5}_{-0.2}, without the use of any prior, and of Gaussian width sigma_g'=1.3^{+0.2}_{-0.7} x 10^15 sqrt{M/0.1M_sun} cm and a ratio sigma_r'/sigma_g'=1.5^{+0.9}_{-0.3} with a prior on the relative velocity between source and microlenses. Both results are in agreement with the predictions of a standard Shakura-Sunyaev disk model. The use of multi-band observations revealed that the magnification event seen in image C of Q2237+0305 was produced by a caustic crossing with a confidence greater than 74%.
Andreas Ernst
Dissolution of Star Clusters in the Galaxy and its Center (thesis pdf, 35 MB) Supervisor:
Andreas Just (ARI)
Thesis abstract: This thesis is concerned with investigations on the dissolution of star clusters in the tidal field of the Galaxy and in particular its center. At first the escape process of stars from star clusters is studied in the framework of chaos-theoretical considerations. Already in the linear tidal approximation it is possible to compute the basins of escape and the chaotic saddle for the system. After the stars have left the star cluster they form tidal arms (or tails) due to the differential rotation of the Galaxy. For star clusters on circular orbits the theoretical framework for the investigation of the properties of tidal arms is discussed. The theory is applied for a star cluster model in the Galactic center. For this purpose a new N-body program called "nbody6gc" has been developed. The algorithm is described in detail and the results of N-body simulations are discussed. At certain positions, well-defined clumps develop in the tidal arms due to the epicyclic motion of the stars. The positions of the clumps are calculated with the analytical theory. Furthermore, a classification of the cluster stars according to radius and specific Jacobi energy is introduced in order to explain the dissolution times and a few results on the "paradox of youth" are formulated.
Martin Hennemann
Multiwavelength observations of massive star-forming regions selected in the far-infrared (thesis pdf, 60 MB) Supervisor:
Dietrich Lemke (MPIA)
Thesis abstract: The lack of observations of the earliest stages in high-mass star formation motivated the selection of massive star-forming regions using the 170 µm ISOPHOT Serendipity Survey. The evaluation of comprehensive follow-up observations, covering near-infrared to (sub-)millimetre wavelengths, identifies massive clumps and characterises the star-forming content in detail. The clumps comprise a large fraction of cold dusty material at temperatures between 12 and 22 K, and several have masses of 100 solar masses or more. Star formation has initiated in every clump, as they harbour embedded sources detected in the mid-infrared that represent low- to intermediate-mass young stellar objects. One case study uses millimetre interferometry and discovers two compact cores of about 15 solar masses embedded in a massive clump. They are driving energetic outflows and may accrete at high rates, and thus represent examples for the first stages of forming intermediate- to high-mass stars. The importance of high spatial resolution in the infrared for the study of high-mass star formation drives dedicated observing programmes with the Herschel Space Observatory. Furthermore, an active part in the preparation of the JWST MIRI instrument was taken because in particular its unprecedented imaging capabilities will allow to constrain crucial properties of the investigated sources.
Alumni - 2008
Pak Hin Tam
(Hong Kong)
Gamma-ray burst studies using the H.E.S.S. Cherenkov array (thesis pdf) Supervisor:
Stefan Wagner (LSW)
Thesis abstract: Gamma-ray bursts (GRBs) are the most intense and unpredictable gamma-ray events from the Universe. Without prior signal, an enormous amount of energy is released for seconds, energizing particles and generating the observed 10e5-10e6 eV gamma-ray photons. The emitting regions can produce Very-High-Energy (VHE) gamma-ray photons of energy about 10e11 eV during and after the burst. These VHE gamma-rays may be attenuated in the source or by the extragalactic background light (EBL). The H.E.S.S. array of four imaging atmospheric Cherenkov telescopes (IACT) is sensitive to VHE gamma-rays. H.E.S.S. observations of 34 GRBs were carried out during 2003-2008, with the shortest response time being six minutes. No evidence of VHE gamma-rays was found. Flux upper limits derived for GRB030329 and GRB060505 are compared and are found consistent with the synchrotron self-Compton flux calculated in the context of relativistic blast-wave model. Absorption by EBL was taken into consideration. Accidentally, GRB060602B was observed with H.E.S.S. throughout the GRB duration, during which no signal of VHE gamma-rays was found. The distance scale and origin of GRB060602B remain unclear and different possibilities and implications are presented. Future prospects of VHE gamma-ray observations of GRBs are discussed
Svitlana Zhukovska
Dust formation by stars and evolution in the interstellar medium Supervisor:
Hans-Peter Gail (ITA)
Thesis abstract: The main goal of this thesis is the study of the origin and evolution of interstellar dust in the Milky Way. We develop a model for the chemical evolution of the galactic disk as a basis for our new model of dust evolution, which considers for the first time the individual evolutions of stardust and of dust condensed in molecular clouds of the Galactic disk. We include dust production by AGB stars in detail, using the results of synthetic AGB models combined with models of dust condensation in stellar outflows, and estimate the efficiency of dust condensation in supernovae by matching model results for the Solar neighbourhood with observed abundances of presolar dust grains of supernova origin. Our results indicate that supernovae produce mainly carbon dust, with only small amounts of silicates, iron and silicon carbonate. We show that the interstellar dust population is dominated by dust grown in the interstellar medium across the Galactic history; moreover, dust formed in AGB stars and supernovae is a dominant source of dust only at metallicities lower than the minimal value for efficient dust growth in molecular clouds.
Giulia Vannoni
Diffusive Shock Acceleration in Radiation Dominated Environments (thesis pdf) Supervisors:
Felix Aharonian /
Werner Hofmann (MPIK)
Thesis abstract: In this work I describe a numerical method developed, for the first time, for the study of Diffusive Shock Acceleration in astrophysical environments where the radiation pressure dominates over the magnetic pressure. This work is motivated by the overwhelming evidence of the acceleration of particles to high energy in astrophysical objects, traced by the non–thermal radiation they emit due to interactions with the gas, radiation fields and magnetic fields. The main objective of this work is to create a generic framework to study self–consistently the interaction of acceleration at shocks and radiative energy losses and the effect such an interplay has on the particle spectrum and on the radiation they emit, in the case when energy losses determine the maximum achievable energy. I apply the developed method to electrons accelerated in three different types of sources: a Supernova Remnant in the Galactic Centre region, a microquasar, and a galaxy cluster. In all three cases the energy losses due to the interaction of electrons with radiation dominate over synchrotron cooling. I demonstrate that there is a strong impact due to the changing features of the inverse Compton scattering from the Thomson to the Klein-Nishina regime, on both the spectrum of accelerated electrons and their broadband emission. I also consider proton acceleration in galaxy clusters, where the particles lose energy during acceleration due to the interaction with the Cosmic Microwave Background radiation. The secondary products from pair production and photomeson processes interact with the same photon field and the background magnetic field, producing broadband electromagnetic radiation from radio to gamma-rays.
Steve Boudreault
The formation of brown dwarfs as revealed by the mass function of IC 2391 (thesis pdf) Supervisor:
Coryn Bailer-Jones (MPIA)
Thesis abstract: This PhD thesis deals with the formation and evolution of brown dwarfs. Here, we present the stellar and substellar mass function of the open cluster IC2391, plus its radial dependence, and use this to put constraints on the formation mechanism of brown dwarfs. Our multiband optical and infrared photometric survey with spectroscopic follow up covers 11 square degrees, making it the largest survey of this cluster to date. We observe that there is no variation in the mass function over the stellar/substellar boundary at all three cluster radius intervals analyzed. From this lack of discontinuity, we conclude that the stellar embryo ejection mechanism cannot be the unique brown dwarf formation path if this formation mechanism produces a higher velocity dispersion for brown dwarfs than the stars obtain. Alternatively, the ejection mechanism could be a dominant brown dwarf formation path only if it produces the same velocity dispersion for brown dwarfs as exists for stars in the cluster. In addition, we observe a radial variation in the mass function over the range 0.15 to 0.5 M_sun. We conclude that this is a signature of mass segregation via dynamical evolution. Analysis of mass functions of other open clusters from different ages and environment, at each side of the stellar/substellar boundary, also indicates that dynamical evolution influences the shape of the mass function.
Oscar Esquivel
Aspects of wave mechanics of gravitating systems (thesis pdf) Supervisor:
Burkhard Fuchs (ARI)
Thesis abstract: The Jeans instability of a galactic disk embedded in a dynamically responsive dark-matter halo is investigated in this work. On small scales the instability is suppressed, if the Toomre stability index Q_T is higher than a certain threshold, but on large scales the Jeans instability sets invariably in. However, using a simple self-consistent disk-halo model it is demonstrated that this occurs on scales which are much larger than the system so that this is indeed only a nominal effect. Also, a rigorous calculation of the dynamical friction (DF) force exerted on a Plummer and a Hernquist sphere moving through an infinite homogenous system of field stars is presented. By using a wave-mechanical treatment, we recover Chandrasekhar's drag force law with a modified Coulomb logarithm that depends on the exact shape of the perturber. We then extend this mode analysis to anisotropic velocity distributions of the field stars. We present easy-to-use handy formulae of the DF force exterted on a point-mass satellite for the cases when the velocity ellipsoid is either oblate or prolate for different values of the effective velocity dispersion sigma_eff determining the anisotropy of the host system.
Michael Zatloukal
A search for distant galaxy clusters (thesis pdf) Supervisor:
Hermann-Josef Röser (MPIA)
Thesis abstract: The evolution of galaxy clusters and their members above z >= 1 is largely unclear at present due to the small number of known distant clusters. To probe this important period and witness the development of key cluster characteristics observed today, the HIROCS survey is establishing a statistically signicant 0.5 <= z <= 1.5 cluster sample. For this thesis, the HIROCS cluster selection function was studied in detail using a mock sky based on semi-analytical galaxy evolution simulations. Public and proprietary data were combined for the COSMOS field. A cluster search was performed, yielding a 0.5 <= z <= 1.59 catalog of clusters, including the irst larger, uniformly selected sample of distant (z > 1.2) clusters published. Comparing the color evolution of X-ray bright and -dark clusters, the latter were found to contain fewer passive galaxies than their X-ray counterparts. Spectroscopic follow-up observations of four z~0.85 candidates, three of them X-ray dark, were conducted to probe cluster detection biases related to this. Using the distant cluster sample, it was shown that z >= 1.2 clusters predominantly contain blue galaxies with active star formation and a significant fraction of interacting members, but that their early type galaxies are old and formed the bulk of their stars at zf >= 3, possibly zf >= 5, in excellent agreement with predictions from simulations.
Irina Golombek
Non-thermal emission of galaxy clusters (thesis pdf) Supervisor:
Matthias Bartelmann (ITA)
Thesis abstract: We investigate the numerical modelling of relativistic protons, so-called cosmic rays, in a magnetised plasma. For the first time we combine two different components of a cosmological simulation code that so far have only been tested and employed independently of each other. By means of magnetohydrodynamic shock tube calculations in a gas that contains a constant fraction of relativistic particles we check for the correct physical behaviour of the combined numerical models. For this purpose we derive an analytical expression for the magnetosonic shock and rarefaction waves in the MHD Riemann problem and solve for the resulting system of equations using an iterative scheme. Comparing the theoretical and numerical solutions of a number of shock tube calculations we assess the physical correctness of the simulation. After successful testing we simulate the structure formation of three galaxy clusters including the consistent modelling of magnetic fields and cosmic rays. By means of analytical models we compute the X-ray and radio emission of the simulated clusters and reveal the temporal correlation between both quantities. Through a comparison with the strong lensing cross sections we demonstrate that the observable radio emission of galaxy clusters is directly connected to the occurrence of substructure in their dark matter halos and is thus triggered by strong merger shocks.
Dominikus Heinzeller
Mass and energy balance in accretion disks (thesis pdf) Supervisor:
Wolfgang Duschl (ITA; Kiel University)
Thesis abstract: We study the role of convection in black hole accretion flows. We investigate the influence of convection on the energy transport as well as the effect of convective turbulence on the disk’s viscosity. The results reveal that convection supports the radiative energy transport efficiently in massless disks, while it can turn into a negative feedback if self-gravity becomes important. Convective turbulence adds significantly to the total viscosity, but cannot account for it on its own. In the second part, we study the spectral energy distribution of super-Eddington accretion flows onto a black hole, based on 2D RHD simulation data. We model the continuum emission as well as the iron K line emission and absorption features with a ray-tracing radiative transfer code. We find that mild relativistic beaming effects become important, leading to super-Eddington luminosities for face-on seen disks. We confirm the diagnostic power of the iron K lines on the accretion process in the inner disk region, finding a strong correlation between the central black hole mass and the ratio of the K-alpha to the K-beta lines. We also detect a trend of line broadening for edge-on seen disks.
Markus Janson
Direct imaging search for substellar companions to nearby stars (thesis pdf) Supervisors:
Wolfgang Brandner / Thomas Henning (MPIA)
Thesis abstract: Ever since the first detections of extrasolar planet candidates in the early 1990s, our knowledge of such objects has drastically increased, and exoplanet science today constitutes a major branch of astrophysics, with a few hundred individual detections. Our physical understanding is however limited by the fact that the planets are generally only detected indirectly, with just a few constraints on its orbital and physical parameters. Direct imaging of exoplanets, where the planet can be spatially resolved from the star, opens up for a much broader understanding of these objects, with the opportunity to study their spectral characteristics. Alternatively, eclipse spectroscopy, where the planetary signal can be temporally resolved in systems where the orbital plane of the planet happens to align with the line of sight of an observer, can be used for the same purpose. In this thesis, we study various approaches for direct imaging and spectroscopy of exoplanets from the ground, using combinations of adaptive optics and differential methods, in particular spectral and angular differential imaging. The possibility of using an external occulter for the purpose of decreasing the star-planet contrast is studied. We also investigate the possibility to calibrate theoretical mass-luminosity relationships in order to improve detection predictions and the interpretations of null-detection surveys. Scientific results include an improved age range of the AB Dor system thanks to the spatial distinguishing of AB Dor B as a close binary, and the best constrained upper limit to the H-band brightness of any known exoplanet ever achieved, from a deep imaging search for Eps Eri b.
Xuepeng Chen
High angular resolution observations of binary protostars (thesis pdf) Supervisors:
Ralf Launhardt / Thomas Henning (MPIA)
Thesis abstract: In this thesis I present a systematic effort to reveal the physical processes that lead to the formation of binary stars. We have observed, at high angular resolution, thirteen isolated low-mass protostellar cores, using the Owens Valley Radio Observatory millimeter array, the Australia Telescope Compact Array, and the IRAM Plateau de Bure Interferometer array. The observations were mainly carried out in the N2H+(1-0) line and at 3mm dust continuum. The results were complemented by infrared data from the Spitzer Space Telescope and the ESO Very Large Telescope. We find that binarity/multiplicity is frequent in the protostellar phase, though it is too early to derive a separation distribution. The circumstellar mass ratio distribution of binary protostars appears to be flat like that of more evolved long-period main-sequence binary stars, and more than 75% of protobinary systems have circumstellar mass ratios below 0.5. The specific angular momenta of protostellar cores are intermediate between those of prestellar cores and the orbital angular momenta of wide pre-main sequence binary systems. There appears to be no significant decrease of angular momentum between the onset of the protostellar collapse and the emergence of a binary star, which suggests that most of the angular momentum contained in the collapse region is transformed into orbital angular momentum of the resulting stellar binary system. We find that during core fragmentation the angular momentum is not evenly, in value and direction, divided between sub-cores. Furthermore, most cores with binary protostars have ratios of rotational to potential gravitational energy of beta_rot > 1%. This is consistent with theoretical simulations and suggests that the initial amount of rotational energy in a molecular cloud core is playing an important role in the protostellar fragmentation process.
Alumni - 2007
Vernesa Smolcic
The faint radio population in the VLA-COSMOS survey: Star forming galaxies and active galactic nuclei (thesis pdf) Supervisors:
Eva Schinnerer / Hans-Walter Rix (MPIA)
Thesis abstract: The composition of the faint (sub-milli-Jansky) radio population, that has been a matter of strong debate in the past, is explored by performing observations at 1.4 GHz (20 cm) radio continuum of the 2 sqdeg COSMOS field providing a large statistically significant sample, and by developing a method that uses a minimal number of parameters to efficiently discriminate between the two main populations in extragalactic radio surveys: active galactic nuclei (AGN) and star forming galaxies. This method bears the potential to be successfully applied to similar samples selected at other wavelengths. One of the main findings is that star forming galaxies do not dominate the submillijansky radio population, as often assumed, but form only about 30-40% of it, while the remainder is composed of AGN and quasars. Using this well defined sample of radio-selected star forming galaxies at 1.4 GHz, the cosmic star formation history is derived using radio data, for the first time constraining the dust-unbiased cosmic evolution of star formation rate in the most intensively star forming galaxies (>100Msun/yr) since ~5 Gyr after the Big Bang with high precision. In addition, the radio derived cosmic star formation history confirms the validity of the large dust corrections applied at other wavelengths.
Slavomir Hnatic
Very High Energy Radiation from Binary System PSR B1259-63/SS2883 (thesis ps.gz) Supervisor:
Felix Aharonian (MPIK)
Thesis abstract: The inverse Compton (IC) scattering of ultrarelativistic electrons accelerated at the pulsar wind termination shock is believed to be responsible for TeV gamma-ray signal reported from the binary system PSR B1259-63/SS2883. While this process can explain the energy spectrum of the observed TeV emission, the gamma-ray fluxes detected by HESS at dif\/ferent epochs do not agree with the published theoretical predictions of the TeV lightcurve. In this work, evolution of the energy spectra of relativistic electrons under different assumptions about the acceleration and energy-loss rates of electrons, is studied. Consequently, it is demonstrated that the observed TeV lightcurve can be explained (i) by adiabatic losses which dominate over the entire trajectory of the pulsar with a signif\/icant increase towards the periastron, or (ii) by sub-TeV cutoffs in the energy spectra of electrons due to the enhanced rate of Compton losses close to the periastron. The calculated spectral and temporal characteristics of the TeV radiation provide conclusive tests to distinguish between these two working hypotheses. The Compton deceleration of the electron-positron pulsar wind contributes to the decrease of the nonthermal power released in the accelerated electrons after the wind termination, and thus to the reduction of the IC and synchrotron components of radiation near the periastron. Although this effect alone cannot explain the observed TeV and X-ray lightcurves, the Comptonization of the cold ultrarelativistic wind leads to the formation of gamma-radiation with a line-type energy spectrum. While the HESS data already constrain the Lorentz factor of the wind, Gamma < 1e6, future observations of this object with GLAST should allow a deep probe of the wind Lorentz factor in the range between 1e4 and 1e6.