The ALMaQUEST Survey: VI. The molecular gas main sequence of `retired' regions in galaxies

TL;DR

This study investigates the molecular gas distribution in star-forming and retired regions of galaxies, revealing that retired regions have significantly less molecular gas and are centrally concentrated, shedding light on galaxy quenching processes.

Contribution

It provides the first detailed comparison of the resolved molecular gas main sequence in retired versus star-forming galaxy regions using ALMaQUEST data.

Findings

Retired regions have a molecular gas main sequence offset to lower Sigma_H2.

Molecular gas is detected in 40-100% of retired spaxels.

Retired regions show up to ten times lower gas fractions than star-forming regions.

Abstract

In order to investigate the role of gas in the demise of star formation on kpc-scales, we compare the resolved molecular gas main sequence (rMGMS: Sigma_* vs Sigma_H2) of star-forming regions to the sequence of `retired' regions that have ceased to form new stars. Using data from the ALMaQUEST survey, we find that retired spaxels form a rMGMS that is distinct from that of star-forming spaxels, offset to lower Sigma_H2 at fixed Sigma_* by a factor of ~5. We study the rMGMS of star-forming and retired spaxels on a galaxy-by-galaxy basis for eight individual ALMaQUEST galaxies. Six of these galaxies have their retired spaxels concentrated within the central few kpc. Molecular gas is detected in 40-100% of retired spaxels in the eight galaxies in our sample. Both the star-forming and retired rMGMS show a diversity in normalization from galaxy-to-galaxy. However, in any given galaxy, the rMGMS for retired regions is found to be distinct from the star-forming sequence and gas fractions of retired spaxels are up to an order of magnitude lower than the star-forming spaxels. We conclude that quenching is associated with a depletion (but not absence) of molecular gas via a mechanism that typically begins in the centre of the galaxy.