Dense gas tracers and star formation laws in active galaxies: Apex survey of HCN J = 4 → 3, HCO+ J = 4 → 3, and CS J = 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 () and that of infrared emission (L IR), up to the highest gas densities (∼106 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 -LIR and -LIR, and a slightly super-linear slope for -LIR. The correlation of -LIR even extends over eight orders of luminosity magnitude down to Galactic dense cores, with a fit of log(LIR) =1.00(± 0.01) ×log() + 4.03(± 0.04). Such linear correlations appear to hold for all densities >104 cm-3, and indicate that star formation rate is not related to the free-fall timescale for dense molecular gas. © 2014. The American Astronomical Society. All rights reserved.