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Published September 2007 | Published
Journal Article Open

The Evolution of the Number Density of Large Disk Galaxies in COSMOS


We study a sample of approximately 16,500 galaxies with I_(ACS,AB) ≤ 22.5 in the central 38% of the COSMOS field, which are extracted from a catalog constructed from the Cycle 12 ACS F814W COSMOS data set. Structural information on the galaxies is derived by fitting single Sérsic models to their two-dimensional surface brightness distributions. In this paper we focus on the disk galaxy population (as classified by the Zurich Estimator of Structural Types), and investigate the evolution of the number density of disk galaxies larger than approximately 5 kpc between redshift z ~ 1 and the present epoch. Specifically, we use the measurements of the half-light radii derived from the Sérsic fits to construct, as a function of redshift, the size function Φ(r_(1/2), z) of both the total disk galaxy population and of disk galaxies split in four bins of bulge-to-disk ratio. In each redshift bin, the size function specifies the number of galaxies per unit comoving volume and per unit half-light radius r_(1/2). Furthermore, we use a selected sample of roughly 1800 SDSS galaxies to calibrate our results with respect to the local universe. We find the following: (1) The number density of disk galaxies with intermediate sizes (r_(1/2) ~ 5-7 kpc) remains nearly constant from z ~ 1 to today. Unless the growth and destruction of such systems exactly balanced in the last eight billion years, they must have neither grown nor been destroyed over this period. (2) The number density of the largest disks (r_(1/2) > 7 kpc) decreases by a factor of about 2 out to z ~ 1. (3) There is a constancy—or even slight increase—in the number density of large bulgeless disks out to z ~ 1; the deficit of large disks at early epochs seems to arise from a smaller number of bulged disks. Our results indicate that the bulk of the large disk galaxy population has completed its growth by z ~ 1 and support the theory that secular evolution processes produce—or at least add stellar mass to—the bulge components of disk galaxies.

Additional Information

© 2007 American Astronomical Society. Print publication: Issue 1 (2007 September); received 2006 April 25; accepted for publication 2006 July 25. We thank the anonymous referee for his valuable comments and suggested improvements to the original manuscript. The HST COSMOS Treasury program was supported through NASA grant HST-GO-09822. We wish to thank Tony Roman, Denise Taylor, and David Soderblom for their assistance in planning and scheduling of the extensive COSMOS observations. We gratefully acknowledge the contributions of the entire COSMOS collaboration consisting of more than 70 scientists. More information on the COSMOS survey is available at http://www.astro.caltech.edu/ ~cosmos. We acknowledge the services provided by the NASA IPAC/IRSA staff in providing online archive and server capabilities for the COSMOS data sets. P. Kampczyk, C. Scarlata, and M. T. Sargent acknowledge support from the Swiss National Science Foundation. Facilities: HST (ACS).

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