Distinguishing the relative contributions of quenching mechanisms is challenging when observing galaxies long after star formation has ceased. However, during and immediately following quenching, these mechanisms imprint distinct spectral and morphological features. Studying galaxies in these transitional stages may, therefore, be the key to understanding the processes that terminate star formation. Post-starburst galaxies (PSBs), are a class of galaxy thought to have undergone a recent, major burst of star formation followed by rapid quenching, and a rare at all epochs.

Potential quenching mechanisms can be broadly separated into two classes: (i) internal processes, such as feedback from star-formation or active galactic nuclei, and (ii) external processes related to the galaxy's environment, such as the stripping of gas in galaxy clusters, due to the pressure exerted on infalling systems from the hot, dense intra-cluster medium. 

During cosmic noon, giant galaxy clusters are still forming from smaller protoclusters, and so environmental quenching is expected to be less efficient at suppressing star formation at higher redshifts. However, in Taylor et al., (2023), we found that galaxy environment influences quenching up to redshifts of at least z ~ 2 in the UDS. Using the growth in the passive population to estimate the fraction of star-forming galaxies quenched per Gyr, we find that the quenching probability increases with both stellar mass and environment, even at these early epochs. We found star-forming galaxies are 1.5 ± 0.1 times more likely to quench in the densest third of environments compared to the lowest density third.

Massive bursts of star formation supply large amounts of energy from supernovae ejecta and radiation, which can generate outflowing galactic-scale winds. High levels of star formation activity are thought to fuel AGN, which can also produce winds capable of rapidly quenching a galaxy. In Taylor et al., (2024), we investigate galactic winds in PSBs at 1 < z < 2, as a function of time since their last starburst. We find evidence for high velocity outflows (v > 1000 km/s) for all but the oldest quenched galaxies, which had their last starburst over one billion years ago. 

The samples show no signs of AGN in optical observations, suggesting that any AGN in these galaxies have very short duty cycles, and were ‘off’ when observed. The presence of significant outflows in the older quenched galaxies is difficult to explain with starburst activity, however, and may indicate energy input from episodic AGN activity as the starburst fades.