2024ApJ...963..115L

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.