Dense gas star formation efficiency and the (Formula presented.) ratio: insights from a statistical study of infrared bright star-forming galaxies

Abstract

We present a statistical study on dense molecular gas tracers of HCN (4–3), HCO+ (4–3) lines and molecular tracers of [C i], and CO observations for a sample of 26 infrared bright star-forming (SF) galaxies. We investigate the dependence of dense gas star formation efficiency traced by HCN (4–3), HCO+ (4–3) (that is (Formula presented.), and (Formula presented.)), and luminosity ratio of (Formula presented.) on [C i]-CO ratios of (Formula presented.), (Formula presented.) and (Formula presented.) (hereafter R[CI]) which are sensitive to interstellar medium conditions. Our findings show that both (Formula presented.) and (Formula presented.) have moderate correlations with (Formula presented.) and R[CI], while (Formula presented.) does not show any significant correlations with any of the [C i]-CO ratios. We compare the (Formula presented.) ratios of AGN and SF galaxies, and find that although the higher (Formula presented.) ratios are mainly found in AGN, the majority of the (Formula presented.) values in SF galaxies are comparable to those in AGN. Based on our findings, it appears that the (Formula presented.) ratio may not be a reliable indicator of the presence of an AGN, although further investigation is needed to confirm this conclusion.