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 Sersic 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 similar to 1 and the present epoch. Specifically, we use the measurements of the half-light radii derived from the Sersic fits to construct, as a function of redshift, the size function Phi( r1/2; 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 r1/2. 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 (r1/2 similar to 5-7 kpc) remains nearly constant from z similar to 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 (r1/2 > 7 kpc) decreases by a factor of about 2 out to z similar to 1. ( 3) There is a constancy-or even slight increase-in the number density of large bulgeless disks out to z similar to 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 similar to 1 and support the theory that secular evolution processes produce-or at least add stellar mass to-the bulge components of disk galaxies.