Structural Evolution of Early-type Galaxies to z = 2.5 in CANDELS

Abstract

Projected axis ratio measurements of 880 early-type galaxies at redshifts 1 < z < 2.5 selected from CANDELS are used to reconstruct and model their intrinsic shapes. The sample is selected on the basis of multiple rest-frame colors to reflect low star-formation activity. We demonstrate that these galaxies as an ensemble are dust-poor and transparent and therefore likely have smooth light profiles, similar to visually classified early-type galaxies. Similar to their present-day counterparts, the z > 1 early-type galaxies show a variety of intrinsic shapes; even at a fixed mass, the projected axis ratio distributions cannot be explained by the random projection of a set of galaxies with very similar intrinsic shapes. However, a two-population model for the intrinsic shapes, consisting of a triaxial, fairly round population, combined with a flat (c/a ~ 0.3) oblate population, adequately describes the projected axis ratio distributions of both present-day and z > 1 early-type galaxies. We find that the proportion of oblate versus triaxial galaxies depends both on the galaxies' stellar mass, and—at a given mass—on redshift. For present-day and z < 1 early-type galaxies the oblate fraction strongly depends on galaxy mass. At z > 1, this trend is much weaker over the mass range explored here (10¹⁰ < M _*/M _⊙ < 10¹¹), because the oblate fraction among massive (M _* ~ 10¹¹ M _⊙) was much higher in the past: 0.59 ± 0.10 at z > 1, compared to 0.20 ± 0.02 at z ~ 0.1. When combined with previous findings that the number density and sizes of early-type galaxies substantially increase over the same redshift range, this can be explained by the gradual emergence of merger-produced elliptical galaxies, at the expense of the destruction of pre-existing disks that were common among their high-redshift progenitors. In contrast, the oblate fraction among low-mass early-type galaxies (log (M _*/M _⊙) < 10.5) increased toward the present, from z = 0 to 0.38 ± 0.11 at z > 1 to 0.72 ± 0.06 at z = 0. We speculate that this lower incidence of disks at early cosmic times can be attributed to two factors: low-mass, star-forming progenitors at z > 1 were not settled into stable disks to the same degree as at later cosmic times, and the stripping of gas from star-forming disk galaxies in dense environments is an increasingly important process at lower redshifts.