2014ApJ...784L..31Z

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 (∼10⁶/cm3) 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'_CS J = 7 - 6_ - L_IR and L'_HCN J =4-3_ - L_IR, and a slightly super-linear slope for L'_HCO+ J = 4-3 - L_IR. The correlation of L'CS J=7-6_ - L_IReven extends over eight orders of luminosity magnitude down to Galactic dense cores, with a fit of log(L_IR) =1.00(± 0.01) xlog(L'_CS J = 7-6) + 4.03(± 0.04). Such linear correlations appear to hold for all densities >10⁴/cm3, and indicate that star formation rate is not related to the free-fall timescale for dense molecular gas.