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

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 Hα/Hβ ratios of face-on QPSBs at z = 0.02-0.15 and with ${M}_{\star }={10}^{10}-{10}^{11}\,{M}_{\odot }$ . We interpret the Hα/Hβ 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 ∼100 PSBs. We estimate the molecular gas by either using PSBs with well-measured Hα/Hβ ratios or measuring them from stacked spectra. Our analysis reveals that QPSBs have a wide range of Hα/Hβ ratios and molecular gas fractions that overlap with the typical gas fractions of star-forming or quiescent galaxies: Hα/Hβ ≈ 3-8 and ${f}_{{{\rm{H}}}_{2}}\approx 1 \% \mbox{--}20 \% $ with median ${f}_{{{\rm{H}}}_{2}}\approx 4 \% \mbox{--}6 \% $ , which correspond to ${M}_{{{\rm{H}}}_{2}}\approx (1\mbox{--}3)\times {10}^{9}\,{M}_{\odot }$ . 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.