What drives galaxy quenching? Resolving molecular gas and star formation in the green valley

We study quenching in seven green valley galaxies on kpc scales by resolving their molecular gas content using 12CO(1–0) observations obtained with NOrthern Extended Millimeter Array and Atacama Large Millimeter Array, and their star formation rate using spatially resolved optical spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory survey. We perform radial stacking of both data sets to increase the sensitivity to molecular gas and star formation, thereby avoiding biases against strongly quenched regions. We find that both spatially resolved gas fraction (fgas) and star formation efficiency (SFE) are responsible for quenching green valley galaxies at all radii: both quantities are suppressed with respect to typical star-forming regions. fgas and SFE have roughly equal influence in quenching the outer disc. We are, however, unable to identify the dominant mechanism in the strongly quenched central regions. We find that fgas is reduced by ∼1 dex in the central regions, but the star formation rate is too low to be measured, leading to upper limits for the SFE. Moving from the outer disc to central regions, the reduction in fgas is driven by an increasing profile rather than a decreasing H2 profile. The reduced fgas may therefore be caused by a decrease in the gas supply rather than molecular gas ejection mechanisms, such as winds driven by active galactic nuclei. We warn more generally that studies investigating fgas may be deceiving in inferring the cause of quenching, particularly in the central (bulge-dominated) regions of galaxies.