Cosmological hydro-dynamical simulations

The formation of nuclear star clusters and other galaxy components such as thick disks, is ultimately tied to the formation history of its host galaxy. Hence, it is necessary to illuminate their connection in the context of hierarchical structure formation in a LambdaCDM universe. Therefore, our group, in collaboration with the Theory Group of Dr. Annalisa Pillepich, analyses state-of-the art cosmological hydrodynamical simulations, such as IllustrisTNG and AURIGA, in order to understand the complex and diverse formation mechanisms of galaxy centers and disk components and provide insights to observational signatures of such systems.

Stellar assembly of galactic nuclei
A simulated galaxy from TNG50 experiences a merger that deposits stars in the host galaxy’s center. The satellite galaxy at z=0.7 is visible towards the top of the host galaxy in the surface gas density (large, middle panel). The tightly bound center of the satellite galaxy is able to withstand the tidal forces of the main galaxy, until it reaches the center of the main galaxy at z=0.35. The lower right panel shows a zoom-in view of the main galaxy’s stellar light; around z=0.44 the nucleus of the merging satellite becomes visible. The lower left panel shows a larger scale view.

We utilize TNG50, the highest resolution installment of the IllustrisTNG simulations, to study the stellar assembly of the central 500 pc of galaxies with stellar masses ranging from 109 to 1012 solar masses. In particular, we divide stars in the center into three different origins:

  1. in-situ stars, which were born in the center
  2. migrated stars, which were born far out in the galaxy disk, but moved to the center
  3. ex-situ stars, which were brought in from the accretion of other galaxies.

While in-situ stars are most prominent, migrated stars can make up as much as 40% to the central galaxy mass. Ex-situ stars are present in the center of almost every galaxy and can contribute over half to the total mass in a galaxy’s center. Migrated stars travel towards the center, either individually or in clusters, from distances of 1-10 kpc or more.

Two key predictions include: 1) Milky Way mass galaxies tend to have an excess of migrated stars, which are on average younger and more rotationally supported than their in-situ stars, 2) high mass star forming galaxies host, in contrast to their quenched counterparts, more ex-situ stars in their centers, which are detectable through their lower metallicities. Thus, galactic centers serve as a laboratory of diverse formation pathways of galaxies, and we present readily observable stellar population and dynamical predictions for these scenarios.

For more information, please contact Alina Böcker.

Thin and thick disks in AURIGA

We analyse the stellar kinematics and populations of galactic thin and thick disks in the AURIGA zoom-in cosmological simulations in the most similar way to what we can do in integral-field spectroscopy observations. We present maps of 27 simulated galaxies seen edge on and the star formation histories of their thin and the thick disks, offering a variety of different cases that is challenging to be achieved in observations of the low-surface brightness thick-disks that require long integration times. We reveal the origin of the different stellar populations going back in time through different simulation snapshots and identifying whether they were formed in situ or ex situ. This analysis will shed light on the origin of thick disks, that is still a matter of debate. We will then provide a recipe to decipher the observed properties of the thin and thick disks in terms of their formation scenarios.

For more information, please contact Francesca Pinna.

Collisional (gravo-thermal) gravitational N-body systems

Unlike collisionless systems, such as the Milky Way galaxy, most star clusters, such as the nuclear star cluster at the centre of our Galaxy, are inherently dynamically collisional (gravo-thermal). For these systems the two-body relaxation time-scale is shorter than the life-time of the system. This simple fact produces many exciting theoretical phenomena, but also computational complications. Consequently, we need massively parallelised software used on the fastest computers (high performance computing GPU clusters) to accurately resolve and couple the small-scale interaction between stars with the global and dynamical evolution of a collisional, gravitational N-body system.

We, in collaboration with Rainer Spurzem's Stellar Dynamics group at the astronomical computation institute (ARI-ZAH) and the National Observatory of the Chinese Academy of Sciences (NAOC-CAS) maintain (among other software) the state-of-the-art direct N-body code Nbody6++GPU, which allows star cluster simulations of realistic size without sacrificing astrophysical accuracy by not properly resolving close binary and/or higher-order subsystems of (degenerate) stars.

Using this, we currently explore the impact of stellar evolution of all kinds of stars (POP-I,-II,-III) on the global evolution of a star cluster across cosmic time (see also Kamlah et al., 2021) and combine this with initial bulk rotation of a star cluster and its impact on the angular momentum transport within a star cluster. We also investigate the impact of initial bulk rotation on the merger rates of stars and compact objects, IMBH formation, as well as the associated gravitational wave merger events.
For more information, please contact Albrecht Kamlah.

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