The Colors and Sizes of Recently Quenched Galaxies: A Result of Compact Starburst before Quenching

Abstract

We analyze the colors and sizes of 32 quiescent (UVJ-selected) galaxies with strong Balmer absorption (EW(Hδ) ≥ 4Å) at z ∼ 0.8 drawn from DR2 of the LEGA-C survey to test the hypothesis that these galaxies experienced compact, central starbursts before quenching. These recently quenched galaxies, usually referred to as post-starburst galaxies, span a wide range of colors and we find a clear correlation between color and half-light radius, such that bluer galaxies are smaller. We build simple toy models to explain this correlation: a normal star-forming disk plus a central, compact starburst component. Bursts with exponential decay timescale of ∼ 100 Myr that produce ∼ 10% to more than 100% of the pre-existing masses can reproduce the observed correlation. More significant bursts also produce bluer and smaller descendants. Our findings imply that when galaxies shut down star formation rapidly, they generally had experienced compact, starburst events and that the large, observed spread in sizes and colors mostly reflects a variety of burst strengths. Recently quenched galaxies should have younger stellar ages in the centers; multi-wavelength data with high spatial resolution are required to reveal the age gradient. Highly dissipative processes should be responsible for this type of formation history. While determining the mechanisms for individual galaxies is challenging, some recently quenched galaxies show signs of gravitational interactions, suggesting that mergers are likely an important mechanism in triggering the rapid shut-down of star-formation activities at z ∼ 0.8.

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

“…Another implication of the scenario proposed is that the youngest PSB galaxies ought to have the smallest half-light radii. This is because the younger central population has a lower mass-to-light ratio, but dims faster than the older, extended population (as indeed confirmed by Wu et al 2020; see their discussion for the effect of dust).…”

“…A possible explanation of the observed properties of PSB galaxies is a central starburst in a previously normal galaxy. A significant amount of star formation inside ≈ 1 kpc from the centre of a galaxy can reduce its previous half-light radius, thus explaining the small observed size of PSB galaxies (Wu et al 2020). At the same time, central starbursts are likely to undergo rapid quenching: either because of strong feedback, or because of the short dynamical time in the central regions of galaxies, which leads to rapid consumption of the cold gas reservoir (e.g.…”

Resolved Multi-element Stellar Chemical Abundances in the Brightest Quiescent Galaxy at z ∼ 2

“…Another issue of key theoretical importance is the link between quenching and bulge formation. Suggested by compaction models, violent disk instabilities funnel gas into galaxy centers, leading to nuclear star formation just prior to galaxy-wide quenching via gas consumption and outflows (e.g., Dekel & Burkert 2014;Zolotov et al 2015;Tacchella et al 2016;Wu et al 2020). However, some observation of quiescent galaxies at z ∼ 2 reported no indication of a bulge (Toft et al 2017), calling into question the generality of this model.…”

Recent insights into massive galaxy formation from observing structural evolution (Review)

“…This suggests that there are distinct and independent physical mechanisms that produce the "rapidly" and "slowly" quenched galaxies. Wu et al (2018) investigated similar questions at z ∼ 0.8 using very deep spectra of massive galaxies collected as part of the LEGA-C survey. They find that, in general, more recently quenched galaxies are larger than older ones.…”