The ALMaQUEST Survey. XII. Dense Molecular Gas as Traced by HCN and HCO + in Green Valley Galaxies

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of two dense gas tracers, HCN (1−0) and HCO + (1-0) for three galaxies in the green valley and two galaxies on the star-forming main sequence with comparable molecular gas fractions as traced by the CO (1−0) emissions, selected from the ALMaQUEST survey. We investigate whether the deficit of molecular gas star formation efficiency (SFE mol ) that leads to the low specific star formation rate (sSFR) in these green valley galaxies is due to a lack of dense gas (characterized by the dense gas fraction f dense ) or the low star formation efficiency of dense gas (SFE dense ). We find that SFE mol as traced by the CO emissions, when considering both star-forming and retired spaxels together, is tightly correlated with SFE dense and depends only weakly on f dense . The sSFR on kiloparsec scales is primarily driven by SFE mol and SFE dense , followed by the dependence on f mol , and is least correlated with f dense or the dense-gas-to-stellar mass ratio ( R dense ). When compared with other works in the literature, we find that our green valley sample shows lower global SFE mol and lower SFE dense while exhibiting similar dense gas fractions when compared to star-forming and starburst galaxies. We conclude that the star formation of the three green valley galaxies with a normal abundance of molecular gas is suppressed, mainly due to the reduced SFE dense rather than the lack of dense gas.