DENSE GAS TRACERS AND STAR FORMATION LAWS IN ACTIVE GALAXIES: APEX SURVEY OF HCN <i>J</i> = 4 → 3, HCO ⁺ <i>J</i> = 4 → 3, AND CS <i>J</i> = 7 → 6

Abstract

We report HCN J = 4 → 3, HCO^+ J = 4 → 3, and CS J = 7 → 6 observations in 20 nearby star-forming galaxies with the Atacama Pathfinder EXperiment 12 m telescope. Combined with four HCN, three HCO^+, and four CS detections from the literature, we probe the empirical link between the luminosity of molecular gas L'_gas and that of infrared emission (L_(IR), up to the highest gas densities (~10^6 cm^(–3)) that have been probed so far. For nearby galaxies with large radii, we measure the IR luminosity within the submillimeter beam size (14"-18") to match the molecular emission. We find linear slopes for L'_(CSJ=7-6)-L_(IR) and L'_(HCNJ=4-3)-L_(IR), and a slightly super-linear slope for L'_(HCO^+J=4-3)-L_(IR). The correlation of L'_(CSJ=7-6)-L_(IR) even extends over eight orders of luminosity magnitude down to Galactic dense cores, with a fit of log(L_(IR)) =1.00(± 0.01)×log L'_(CSJ=7-6) + 4.03(± 0.04). Such linear correlations appear to hold for all densities &gt;10^4 cm^(–3), and indicate that star formation rate is not related to the free-fall timescale for dense molecular gas.