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Fig. 1: Examples of surface brightness model fits to 3 disk galaxies (original image in leftmost, residual map in right-most column). Image sizes are 10''.





Fig. 2: Evolution of the bivariate size-luminosity function of bulgeless (top) & bulge-dominated (bottom) disk galaxies.





Fig. 3: Surface brightness evolution of COSMOS disk galaxy samples compared to previous work.





Fig. 4: Evolution of COSMOS disk galaxies of varying B/D-ratio with evolutionary predictions for synthetic stellar opulations.

Observational studies of disk galaxies from high to low redshifts are important in two ways:

  1. They can provide observational constraints to theoretical models and simulations, and
  2. directly shed light on the sequence of events which has progressively formed the galaxy population observed today.
On the observational frontier evidence has been accumulating in the past years that disk galaxies have led a relatively quiet life in the past eight billion years. Recent look-back surveys have indicated that
  • the number density of large disk galaxies has not changed significantly since z ~ 1 (e.g. Lilly, 1998),
  • the stellar mass-size relation has not evolved since z ~ 1 while the average surface brightness has declined by somewhat more than 1 mag over the same time (e.g. Barden, 2005), and
  • the mass function of late-type galaxies has not experienced significant changes either in the period (e.g. Brinchmann & Ellis, 2001).
It thus appears as if the main process governing disk galaxy evolution since z ~ 1 was the aging of their stellar populations.

In addition to this, internal transformations have most likely played an important role in shaping the Hubble sequence as well. Redistribution within galactic disks of material and angular momentum by bar instabilities or other disk dynamical features are a likely cause of bulge formation in the later life of disk galaxies. These processes are collectively referred to as 'secular evolution' (e.g Kormendy & Kennicutt, 2004; Carollo, 2004) and gradually changed the relative numbers of pure and bulged disk galaxies in the past few billions of years.

1 This emerging consenus on the transformations in disk galaxies is the outcome of significant controversy in the past decade concerning both the evolution of surface brightness in the disk galaxy population as well as the evolution of disk galaxy sizes during the past half of the Hubble time.

The study of the evolution of the disk galaxy population since redshift z ~ 1, (i.e. during the last eight billion years) has been and continues to be one of the major focuses of my research. My work on this subject is primarily based on data from the Cosmic Evolution Survey (COSMOS).
COSMOS is currently the largest look-back survey with data from the radio wave band to X-rays acquired in a 2 deg2 equatorial field (about ten times the area of the moon) in order to study the evolution of the galaxies as a function of environment since z ~ 3. The program includes the largest single observational Hubble Space Telescope (HST) project ever carried out. Thanks to the high resolution and the depth of the I-band HST images (the pixel scale of the space-based exposures is 0.4 kpc at z ~ 1) the morphological properties of galaxies at optical wavelengths can be measured for galaxies observed when the universe was less than half as old as it is today.

To measure galactic structure for intermediate redshift galaxies imaged with HST we used two techniques. Following the so-called 'parametric' approach to quantify morphology, an analytic law - the Sérsic profile - is fitted to the projected two-dimensional surface brightness distribution of galaxies observed in the COSMOS field (see Fig. 1). This is complemented by a 'non-parametric' classification scheme, the 'Zurich Estimator of Structural Types' (ZEST), in which measures of concentration, asymmetry, clumpiness, etc. are used to separate early-type from disk and irregular galaxies. Disk galaxies are further split into different categories according to the bulge-to-disk (B/D) ratio.

The COSMOS survey makes it possible to address unresolved questions concerning the evolution of disk galaxies - such as the evolution of surface brightness, size, etc. - with a data set that is signicantly larger and su ffers significantly less from cosmic sampling variance than any sample employed in previous reports. Furthermore, the large statistical sample provided by COSMOS, and the large volumes of the survey, covering all scales of the large scale structure at all redshifts of relevance for our work, allow us to investigate the dependence of evolutionary trends in the disk galaxy population on bulge-to-disk ratio, and environmental density.

In a first project carried out while HST was still gathering data in the COSMOS field we studied the evolution of the size function of large disk galaxies between z ~ 1 and the present using a sample of more than 11,000 disk galaxies with I < 22.5, extracted from the central 38% of the COSMOS field (see Sargent et al., 2007).
With the completion of the high-resolution imaging in space it was then possible as the next step to investigate the evolution of the bivariate size-luminosity function and of the opacity and surface brightness of large disk galaxies at z < 1. This analysis is based on the complete sample of more than 27,500 disk galaxies present over the entire area of the COSMOS field. At the same time we also investigated the history of star formation in disk galaxies, as well as the eff ects of environment on the detected evolutionary trends.

The most important findings that have emerged from this work are that
  1. On average, the bulk of the large disk galaxy population had completed its growth by z ~ 1. However, bulgeless disks were more frequent at high redshift than nowadays, while the fraction of bulge-dominated disks has grown steadily since z ~ 1 (Fig. 2). Processes of secular evolution have built up - or at least added stellar mass to - the bulge components of disk galaxies over the same period.

  2. The typical sizes of bulgeless disk galaxies have hardly changed over the past eight billion years yet the typical sizes of bulge-dominated galaxies have increased by about 30% over this time span (Fig. 2). The changes in the relative abundance of bulged and bulgeless disks conspire to produce a weaker evolution in surface brightness in the disk galaxy population as a whole in comparison with the individual B/D classes (Fig. 3).

  3. The star formation history in the disk component of disk galaxies does not depend on the size of the bulge that is embedded in the disk at the centre of the galaxy (Fig. 4).

  4. Surface brightness and luminosity evolution are independent of environment. Nevertheless, environment may a ffect the evolution of disk galaxies in that the colour evolution of disk galaxies becomes progressively weaker the denser the environment.

Many more details on both the morphological classification of large samples of distant galaxies and its application to the study of disk galaxy evolution can be found here:
  • 'The Evolution of Disk Galaxies in the COSMOS-Survey' (Ph.D. thesis)
    M. Sargent (supervision by M. Carollo), 2007, ETHZ
    Download (19.7 MB):

  • Soon to come: 'The role of environment in the growth of disk galaxies: surface brightness, size & star formation histories''
    Sargent, M. T., Carollo, C. M., Lilly, S. J., Oesch, P. et al. 2010, in prep.

  • 'The Evolution of the Number Density of Large Disk Galaxies in COSMOS'
    Sargent, M. T., Carollo, C. M., Lilly, S. J., Scarlata, C. et al. 2007, ApJS, 172, 434

  • 'COSMOS Morphological Classification with the Zurich Estimator of Structural Types (ZEST) and the Evolution Since z = 1 of the Luminosity Function of Early, Disk, and Irregular Galaxies'
    Scarlata, C., Carollo, C. M., Lilly, S., Sargent, M. T. et al. 2007, ApJS, 172, 406


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M. Sargent :: November 2008