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

We study a sample of approximately 16,500 galaxies with I_AB <= 22.5 in theCOSMOS field. Structural information on the galaxies is derived by fittingsingle Sersic models to their two-dimensional surface brightness distributions.We investigate the evolution of the number density of disk galaxies larger than5 kpc between redshift z~1 and the present epoch. To this end, we use themeasurements of the half-light radii to construct, as a function of redshift,the size function of both the total disk galaxy population and of disk galaxiessplit in four bins of bulge-to-disk ratio. Furthermore, we use a selectedsample of roughly 1800 SDSS galaxies to calibrate our results with respect tothe local universe. We find that: (i) The number density of disk galaxies withintermediate sizes (r_{1/2}~5-7 kpc) remains nearly constant from z~1 to today.(ii) The number density of the largest disks (r_{1/2}>7 kpc) decreases by afactor of about two out to z~1. (iii) There is a constancy in the numberdensity of large bulgeless disks out to z~1; the deficit of large disks atearly epochs seems to arise from a smaller number of bulged disks. Our resultsindicate that the bulk of the large disk galaxy population has completed itsgrowth by z~1, and support the hypothesis that secular evolution processesproduce - or at least add stellar mass to - the bulge components of diskgalaxies.