Some Die Filthy Rich: The Diverse Molecular Gas Contents of Post-starburst Galaxies Probed by Dust Absorption

TL;DR

This study investigates the molecular gas content of post-starburst galaxies, revealing a wide range of gas fractions that challenge previous assumptions about their gas depletion and the effects of feedback mechanisms.

Contribution

It applies a new method to a larger, uniformly selected sample of post-starburst galaxies to accurately estimate their molecular gas content using Hα/Hβ ratios.

Findings

QPSBs have diverse molecular gas fractions overlapping with star-forming and quiescent galaxies.

Large cold gas reservoirs are present in many QPSBs, contradicting the idea of complete gas removal.

Feedback from active galactic nuclei does not necessarily destroy all cold gas in QPSBs.

Abstract

Quenched post-starburst galaxies (QPSBs) are a rare but important class of galaxies that show signs of rapid cessation or recent rejuvenation of star formation. A recent observation shows that about half of QPSBs have large amounts of cold gas. This molecular CO sample is, however, too small and is not without limitations. Our work aims to verify previous results by applying a new method to study a uniformly selected sample, more than 10 times larger. In particular, we present detailed analysis of H$\alpha$/H$\beta$ ratios of face-on QPSBs at $z = 0.02 - 0.15$ and with $M_\star = 10^{10}-10^{11}\,M_\odot$. We interpret the H$\alpha$/H$\beta$ ratios by applying our recent gas mass calibration, which is based on non-PSB galaxies but predicts gas masses that are consistent with CO observations of $\sim 100$ PSBs. We estimate the molecular gas by either using PSBs with well-measured H$\alpha$/H$\beta$ ratios or by measuring them from stacked spectra. Our analysis reveals that QPSBs have a wide range of H$\alpha$/H$\beta$ ratios and molecular gas fractions that overlap with the typical gas fractions of star-forming or quiescent galaxies: H$\alpha$/H$\beta \approx 3-8$ and $f_\mathrm{H_2} \approx 1\%-20\%$ with median $f_\mathrm{H_2} \approx 4\%-6\%$, which correspond to $M_\mathrm{H_2} \approx (1-3) \times 10^{9} \,M_\odot$. Our results indicate that large reservoirs of cold gas are still present in significant numbers of QPSBs, and that they arguably were not removed or destroyed by feedback from active galactic nuclei.