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.

Keywords

• Galaxy evolution
• Green valley galaxies
• Molecular gas