Inverse stellar population age gradients of post-starburst galaxies at z = 0.8 with LEGA-C

Abstract

We use deep, spatially resolved spectroscopy from the LEGA-C Survey to study radial variations in the stellar population of 17 spectroscopically-selected post-starburst (PSB) galaxies. We use spectral fitting to measure two Lick indices, HδA and Fe4383, and find that, on average, PSB galaxies have radially decreasing HδA and increasing Fe4383 profiles. In contrast, a control sample of quiescent, non-PSB galaxies in the same mass range shows outwardly increasing HδA and decreasing Fe4383. The observed gradients are weak (≈−0.2Å/Re), mainly due to seeing convolution. A two-SSP model suggests intrinsic gradients are as strong as observed in local PSB galaxies (≈−0.8Å/Re). We interpret these results in terms of inside-out growth (for the bulk of the quiescent population) vs star formation occurring last in the centre (for PSB galaxies). At z ≈ 0.8, central starbursts are often the result of gas-rich mergers, as evidenced by the high fraction of PSB galaxies with disturbed morphologies and tidal features (40%). Our results provide additional evidence for multiple paths to quiescence: a standard path, associated with inside-out disc formation and with gradually decreasing star-formation activity, without fundamental structural transformation, and a fast path, associated with centrally-concentrated starbursts, leaving an inverse age gradient and smaller half-light radius.

Searching for local counterparts of high-redshift post-starburst galaxies in integral field unit spectroscopic surveys of nearby galaxies

“…They also have flatter color gradient than older quiescent galaxies (Suess et al 2020). The light and color profiles can be explained by a centrally-concentrated star-formation activity happend before quenching (Wu et al 2020). Simulations show that highly dissipative processes induced by galaxy mergers and interactions are able to drive gas into galaxies centers and invoke compact starbursts then quenching (Bekki et al 2005;Wild et al 2009;Snyder et al 2011;Zheng et al 2020).…”

“…While photometric selections are now prevalent in high-redshift studies (e.g. Whitaker et al 2012), spectroscopical features are potentially more sensitive to the rapid decline of star-formation rates when high-S/N spectra are available (Wu et al 2020).…”

“…Several studies reported relatively small half-light radii of high-z post-starburst galaxies and in general attributed to compact starburst events triggered by highly-dissipative processes (Whitaker et al 2012;Belli et al 2015;Wu et al 2018). Wu et al (2020) specifically proposed that the smaller half-light radii is a result of a centrally-concentrated starburst event on top of an aged population. The most compact post-starburst galaxies (R e 1 kpc) are those doubled the stellar masses in the starburst events.…”

“…In principle, these predictions can be tested and verified by the spatially-resolved continuum spectra of post-starburst galaxies. Nevertheless, collecting spatially-resolved continuum spectra for galaxies beyond the nearby Universe is extremely difficult: long integration time on large telescopes with good seeing (D'Eugenio et al 2020;Setton et al 2020) and preferentially with the assist of adaptive optics or gravitational lenses to resolve the galaxies. Furthermore, the principal driving mechanisms likely depend on both the stellar mass and the environment (Wu et al 2014;Maltby et al 2018;Matharu et al 2020;Setton et al 2020).…”

The Large Early Galaxy Astrophysics Census (LEGA-C) Data Release 3: 3000 High-quality Spectra of K s -selected Galaxies at z > 0.6

“…At the same time, the question whether galaxies change their structure upon 'quenching' -the transition from an actively star-forming state to a quiescent state -remains open. Wu et al (2020) andD'Eugenio et al (2020) show that post-starburst galaxies rapidly formed a large number of stars in the central regions of galaxies, producing compact galaxies. But, at the same time, larger galaxies are generally younger (Wu et al 2018a), reflecting a complexity in galaxy evolution that can only be re-vealed by high-quality spectroscopic data.…”