We present ALMA observations of dense gas tracers HCN (1−0) and HCO+ (1-0) for three green valley galaxies and two galaxies on the star-forming main sequence, all with comparable molecular gas fractions as traced by CO (1−0) emissions from the ALMaQUEST survey. Our study examines whether the low molecular gas star formation efficiency (SFE_mol) in green valley galaxies is due to a lack of dense gas or low star formation efficiency of dense gas (SFE_dense). We find that SFEmol is tightly correlated with SFEdense and only weakly depends on the dense gas fraction (f_dense). Compared to other studies, our green valley sample shows lower global SFE_mol and SFE_dense but similar dense gas fractions to star-forming and starburst galaxies. We conclude that the reduced SFEdense, rather than a lack of dense gas, primarily suppresses star formation in these green valley galaxies. (Lihwai Lin et al. 2024, ApJ, 963, 115)
