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

TL;DR

This study analyzes the 3D shapes of galaxies at redshifts 0.2 to 1.0, revealing how their morphology and thickness evolve with star formation activity, stellar mass, and cosmic time using HST data.

Contribution

It provides the first statistical estimation of intrinsic 3D galaxy shapes across a large sample at intermediate redshifts, linking morphological evolution to star formation quenching.

Findings

Transition from thin disk to thick spheroid at specific star formation rate ~-1 dex.

Passive galaxies become thinner over time from z~0.8 to z~0.4.

Massive star-forming galaxies tend to have thinner shapes than less massive ones.

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 $z<1$.