Central Concentration of Asymmetric Features in Post-starburst Galaxies at $z \sim 0.8$

Abstract

We present morphological analyses of Post-starburst galaxies (PSBs) at $0.7<z<0.9$ in the COSMOS field. We fitted ultraviolet to mid-infrared multi-band photometry of objects with $i<24$ from COSMOS2020 catalogue with population synthesis models assuming non-parametric, piece-wise constant function of star formation history, and selected 94 those galaxies that have high specific star formation rates (SSFRs) of more than $10^{-9.5}$ yr$^{-1}$ in 321--1000 Myr before observation and an order of magnitude lower SSFRs within recent 321 Myr. We devised a new non-parametric morphological index which quantifies concentration of asymmetric features, $C_{A}$, and measured it as well as concentration $C$ and asymmetry $A$ on the Hubble Space Telescope/Advanced Camera for Surveys $I_{\rm F814W}$-band images. While relatively high $C$ and low $A$ values of PSBs are similar with those of quiescent galaxies rather than star-forming galaxies, we found that PSBs show systematically higher values of $C_{A}$ than both quiescent and star-forming galaxies; 36% of PSBs have $\log{C_{A}} > 0.8$, while only 16% (2%) of quiescent (star-forming) galaxies show such high $C_{A}$ values. Those PSBs with high $C_{A}$ have relatively low overall asymmetry of $A \sim 0.1$, but show remarkable asymmetric features near the centre. The fraction of those PSBs with high $C_{A}$ increases with increasing SSFR in 321--1000 Myr before observation rather than residual on-going star formation. These results and their high surface stellar mass densities suggest that those galaxies experienced a nuclear starburst in the recent past, and processes that cause such starbursts could lead to the quenching of star formation through rapid gas consumption, supernova/AGN feedback, and so on.