The ALMaQUEST Survey XII: Dense Molecular Gas as traced by HCN and HCO${}^{+}$ in Green Valley Galaxies

Abstract

We present 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${}_{\mathrm{mol}}$) that leads to the low specific star formation rate in these green valley galaxies is due to a lack of dense gas (characterized by the dense gas fraction $f_{\mathrm{dense}}$) or the low star formation efficiency of dense gas (SFE${}_{\mathrm{dense}}$). We find that SFE${}_{\mathrm{mol}}$ as traced by the CO emissions, when considering both star-forming and retired spaxels together, is tightly correlated with SFE${}_{\mathrm{dense}}$ and depends only weakly on $f_{\mathrm{dense}}$. The specific star formation rate (sSFR) on kpc scales is primarily driven by SFE${}_{\mathrm{mol}}$ and SFE${}_{\mathrm{dense}}$, followed by the dependence on $f_{\mathrm{mol}}$, and is least correlated with $f_{\mathrm{dense}}$ or the dense-to-stellar mass ratio ($R_{\mathrm{dense}}$). When compared with other works in the literature, we find that our green valley sample shows lower global SFE${}_{\mathrm{mol}}$ as well as lower SFE${}_{\mathrm{dense}}$ while exhibiting similar dense gas fractions when compared to star-forming and starburst galaxies. We conclude that the star formation of the 3 green valley galaxies with a normal abundance of molecular gas is suppressed mainly due to the reduced SFE${}_{\mathrm{dense}}$ rather than the lack of dense gas.

20 pages, 13 figures, ApJ accepted