Cold gas in a post-starburst pair at z ~ 1.4: major mergers as a pathway to quenching in the HeavyMetal survey

TL;DR

This study investigates molecular gas in post-starburst galaxies at z ~ 1.4, revealing that major mergers may help retain gas while suppressing star formation, challenging previous low-redshift findings.

Contribution

It provides new ALMA CO observations of high-redshift post-starburst galaxies, highlighting the role of major mergers in galaxy quenching and cold gas retention.

Findings

One galaxy detected with significant molecular gas in a merger system.

Most galaxies show no CO detection, indicating low gas content.

No correlation between gas mass and time since quenching at z ~ 1.4.

Abstract

Recent observations at low redshift have revealed that some post-starburst galaxies retain significant molecular gas reservoirs despite low ongoing star formation rates, challenging theoretical predictions for galaxy quenching. To test whether this finding holds during the peak epoch of quenching, here we present ALMA CO(2-1) observations of five spectroscopically confirmed post-starburst galaxies at z ~ 1.4 from the HeavyMetal survey. While four galaxies are undetected in CO emission, we detect M_H2 ~ 10^9.7 Msun of molecular gas in one system. The detected system is a close pair of massive (M* = 10^(11.1-11.2) Msun) post-starburst galaxies with no clear tidal features, likely caught in the early stages of a major merger. These results suggest that mergers may be a key factor in retaining molecular gas while simultaneously suppressing star formation in quenched galaxies at high redshift, possibly by driving increased turbulence that decreases star formation efficiency. Unlike previous studies at z < 1, we find no correlation between molecular gas mass and time since quenching. This may be explained by the fact that -- despite having similar UVJ colors -- all galaxies in our sample have post-burst ages older than typical gas-rich quenched systems at low redshift. Our results highlight the importance of major mergers in shaping the cold gas content of quiescent galaxies during the peak epoch of quenching.