The Majority of Compact Massive Galaxies at z ~ 2 are Disk Dominated

Abstract

We investigate the stellar structure of massive, quiescent galaxies at z ~ 2, based on Hubble Space Telescope/WFC3 imaging from the Early Release Science program. Our sample of 14 galaxies has stellar masses of M _* > 10^10.8 M _sun and photometric redshifts of 1.5 < z < 2.5. In agreement with previous work, their half-light radii are <2 kpc, much smaller than equally massive galaxies in the present-day universe. A significant subset of the sample appears highly flattened in projection, which implies, considering viewing angle statistics, that a significant fraction of the galaxies in our sample have pronounced disks. This is corroborated by two-dimensional surface brightness profile fits. We estimate that 65% ± 15% of the population of massive, quiescent z ~ 2 galaxies are disk dominated. The median disk scale length is 1.5 kpc, substantially smaller than the disks of equally massive galaxies in the present-day universe. Our results provide strong observational evidence that the much-discussed ultra-dense high-redshift galaxies should generally be thought of as disk-like stellar systems with the majority of stars formed from gas that had time to settle into a disk.