Relation between colour gradient and central asymmetric features for post-starburst galaxies at $z \sim 0.8$

Abstract

We investigated colour gradients of photometrically selected post-starburst galaxies (PSBs) at $0.7 < z < 0.9$ in the COSMOS field as a function of central concentration of asymmetric features, $C_{A}$, in order to understand their origins. We measured the colour gradients for 33 PSBs, 332 quiescent galaxies (QGs), and 1136 star-forming gaaxies (SFGs) by using COSMOS $HST$/ACS $I_{F814W}$-band and COSMOS-DASH $HST$/WFC3 $H_{F160W}$-band data. We found that the colour gradient, $\Delta (I-H) = (I-H)_{in} - (I-H)_{out}$, decreases with increasing $C_{A}$ for all the three populations. Only PSBs with $\log{C_{A}} > 0.6$ show positive gradients, which suggests that their central asymmetric features are caused by disturbed distribution of relatively young stars near the centre. The colour gradients are also closely related with half-light radius rather than stellar mass for all the populations. The positive colour gradients and very small sizes of those PSBs with high $C_A$ suggest that a nuclear starburst caused by gas-rich major merger occurred in the recent past. On the other hand, similarly massive PSBs with $\log C_A < 0.6$ show the negative colour gradients, heavier dust extinction, and larger sizes, and their origins may be different from those PSBs with high $C_A$.