Evolution of the Three-dimensional Shape of Passively Evolving and Star-forming Galaxies at z < 1

Abstract

Using the HST/ACS I _F814W-band data, we investigated the distribution of apparent axial ratios of ∼21,000 galaxies with M _V < -20 at 0.2 < z < 1.0 in the COSMOS field as a function of stellar mass, specific star formation rate (sSFR), and redshift. We statistically estimated intrinsic 3D shapes of these galaxies by fitting the axial-ratio distribution with triaxial ellipsoid models characterized by face-on (middle-to-long) and edge-on (short-to-long) axial ratios B/A and C/A. We found that the transition from thin disk to thick spheroid occurs at ΔMS ∼ -1 dex, i.e., 10 times lower sSFR than that of the main sequence for galaxies with M _star = 10¹⁰-10¹¹ M _⊙ at 0.2 < z < 1.0. Furthermore, the intrinsic thickness (C/A) of passively evolving galaxies with M _star = 10¹⁰-10¹¹ M _⊙ significantly decreases with time from C/A ∼ 0.40-0.50 at z ∼ 0.8 to C/A ∼ 0.33-0.37 at z ∼ 0.4, while those galaxies with M _star > 10¹¹ M _⊙ have C/A ∼ 0.5 irrespective of redshift. On the other hand, star-forming galaxies on the main sequence with 10^9.5-10¹¹ M _⊙ show no significant evolution in their shape at 0.2 < z < 1.0, but their thickness depends on stellar mass; more massive star-forming galaxies tend to have lower C/A (thinner shape) than low-mass ones. These results suggest that some fraction of star-forming galaxies with a thin disk, which started to appear around z ∼ 1, quench their star formation without violent morphological change, and these newly added quiescent galaxies with a relatively thin shape cause the significant evolution in the axial-ratio distribution of passively evolving galaxies with M _star < 10¹¹ M _⊙ at z < 1.