Central concentration of asymmetric features in post-starburst galaxies at z 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 multiband 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 of 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_F814W-band images. While relatively high C and low A values of PSBs are similar to 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 per cent of PSBs have log C_A > 0.8, while only 16 per cent (2 per cent) of quiescent (star-forming) galaxies show such high C_A values. Those PSBs with high C_A have relatively low overall asymmetry of A ~ 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 ongoing 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/active galactic nucleus feedback, and so on.