The structure of post-starburst galaxies at 0.5 < z < 2: evidence for two distinct quenching routes at different epochs

We present an analysis of the structure of post-starburst (PSB) galaxies in the redshift range 0.5 < z < 2, using a photometrically selected sample identified in the Ultra Deep Survey field. We examine the structure of ∼80 of these transient galaxies using radial light μ(r) profiles obtained from CANDELS Hubble Space Telescope near-infrared/optical imaging, and compare to a large sample of ∼2000 passive and star-forming galaxies. For each population, we determine their typical structural properties (effective radius r_e, Sérsic index n) and find significant differences in PSB structure at different epochs. At high redshift (z > 1), PSBs are typically massive (M_* > 10^{10} M_{⊙}), very compact and exhibit high Sérsic indices, with structures that differ significantly from their star-forming progenitors but are similar to massive passive galaxies. In contrast, at lower redshift (0.5 < z < 1), PSBs are generally of low mass (M_* < 10^{10} M_{⊙}) and exhibit compact but less concentrated profiles (i.e. lower Sérsic indices), with structures similar to low-mass passive discs. Furthermore, for both epochs, we find remarkably consistent PSB structure across the optical/near-infrared wavebands (which largely trace different stellar populations), suggesting that any preceding starburst and/or quenching in PSBs was not strongly centralized. Taken together, these results imply that PSBs at z > 1 have been recently quenched during a major disruptive event (e.g. merger or protogalactic collapse) that formed a compact remnant, while at z < 1 an alternative less disruptive process is primarily responsible. Our results suggest that high-z PSBs are an intrinsically different population to those at lower redshifts, and indicate different quenching routes are active at different epochs.