Evolution of 3-dimensional Shape of Passively Evolving and Star-forming Galaxies at $z<1$

Abstract

Using the HST/ACS $I_{\rm F814W}$-band data, we investigated distribution of apparent axial ratios of $\sim21000$ 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 3-dimensional 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 $\Delta$MS $\sim-1$ dex, i.e., 10 times lower sSFR than that of the main sequence for galaxies with $M_{\rm star} = 10^{10}$--$10^{11} M_{\odot}$ at $0.2<z<1.0$. Furthermore, the intrinsic thickness ($C/A$) of passively evolving galaxies with $M_{\rm star}=10^{10}$--$10^{11}M_{\odot}$ significantly decreases with time from $C/A \sim 0.40$ -- $0.50$ at $z\sim 0.8$ to $C/A\sim0.33$ -- $0.37$ at $z\sim0.4$, while those galaxies with $M_{\rm star}>10^{11}M_{\odot}$ have $C/A\sim0.5$ irrespective of redshift. On the other hand, star-forming galaxies on the main sequence with $10^{9.5}$--$10^{11}M_{\odot}$ 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 $z\sim1$, 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 $M_{\rm star}<10^{11}M_{\odot}$ at $z<1$.