Now you see it, now you don't: Star formation truncation precedes the loss of molecular gas by ~100 Myr in massive post-starburst galaxies at z~0.6

Abstract

We use ALMA observations of CO(2-1) in 13 massive ($M_{\star}\gtrsim 10^{11} M_{\odot}$) post-starburst galaxies at $z\sim0.6$ to constrain the molecular gas content in galaxies shortly after they quench their major star-forming episode. The post-starburst galaxies in this study are selected from the Sloan Digital Sky Survey spectroscopic samples (DR14) based on their spectral shapes, as part of the SQuIGGLE program. Early results showed that two post-starburst galaxies host large H$_2$ reservoirs despite their low inferred star formation rates. Here we expand this analysis to a larger statistical sample of 13 galaxies. Six of the primary targets (45%) are detected, with $M_{H_2}\gtrsim10^9 M_{\odot}$. Given their high stellar masses, this mass limit corresponds to an average gas fraction of $\langle f_{H_2} \equiv M_{H_2}/M_{\star} \rangle \sim7\%$, or ${\sim}14\%$ using lower stellar masses estimates derived from analytic, exponentially declining star formation histories. The gas fraction correlates with the $D_n4000$ spectral index, suggesting that the cold gas reservoirs decrease with time since burst, as found in local K+A galaxies. Star formation histories derived from flexible stellar population synthesis modeling support this empirical finding: galaxies that quenched $\lesssim 150$ Myr prior to observation host detectable CO(2-1) emission, while older post-starburst galaxies are undetected. The large $\mathrm{H_2}$ reservoirs and low star formation rates in the sample imply that the quenching of star formation precedes the disappearance of the cold gas reservoirs. However, within the following 100-200 Myrs, the SQuIGGLE galaxies require the additional and efficient heating or removal of cold gas to bring their low star formation rates in line with standard $\mathrm{H_2}$ scaling relations.