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

TL;DR

This study analyzes the morphology of post-starburst galaxies at z~0.8, revealing that many exhibit concentrated asymmetric features near their centers, indicative of recent nuclear starbursts potentially linked to star formation quenching.

Contribution

Introduces a new non-parametric morphological index, $C_{A}$, to quantify asymmetric features in PSBs and links these features to recent nuclear starburst activity.

Findings

High $C_{A}$ values are more common in PSBs with higher past SSFRs.

Many PSBs show central asymmetric features despite low overall asymmetry.

High surface stellar mass densities suggest recent nuclear starbursts in PSBs.

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.