- Ilsang Yoon
- Yancy Shirley
- Yujin Yang
- Adam Smercina
- 'J.D. Smith'
- Desika Narayanan
bibliography:
- 'earefs.bib'
title: 'Why Post-Starburst Galaxies Are Now Quiescent'
---

Introduction
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The nature of how and when the molecular gas reservoirs are depleted in galaxies is essential to understanding the question of why galaxies become quiescent. To explore this question, we focus on a class of galaxies in the midst of rapid evolution in their star formation properties. The spectra of post-starburst, “E+A", or “k+a" galaxies show strong Balmer absorption, indicative of a recent starburst that ended in the last Gyr, yet little ongoing star formation, indicating a rapid change from star-forming to quiescent [@Dressler1983; @Couch1987]. Despite their low star formation rates (SFRs), many (over half of those studied) have large CO-traced molecular gas reservoirs [@French2015; @Rowlands2015; @Alatalo2016b]. These quiescent post-starburst galaxies have similar CO-traced molecular gas fractions (M$_{\rm mol}/$M$_\star$) as normal star-forming galaxies, implying a lower CO-traced star formation efficiency SFE $\propto$ SFR/$L^{\prime}($CO$)$. This offset persists in the classical Kennicutt-Schmidt [@Kennicutt1998] relation, suggesting post-starburst galaxies experience a $\sim 4 \times$ suppression of SFE in the CO-traced molecular gas.

Previous studies of the molecular gas content of post-starburst galaxies have 