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  = 10 10 –10 11 M ⊙ at 0.2 <  z  < 1.0. Furthermore, the intrinsic thickness ( C / A ) of passively evolving galaxies with M star  = 10 10 –10 11 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  > 10 11 M ⊙ have C / A  ∼ 0.5 irrespective of redshift. On the other hand, star-forming galaxies on the main sequence with 10 9.5 –10 11 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  < 10 11 M ⊙ at z  < 1.