Testing the universality of the star-formation efficiency in dense molecular gas

Context. Recent studies with, for example, Spitzer and Herschel have suggested that star formation in dense molecular gas may be governed by essentially the same “law” in Galactic clouds and external galaxies. This conclusion remains controversial, however, in large part because different tracers have been used to probe the mass of dense molecular gas in Galactic and extragalactic studies. Aims. We aimed to calibrate the HCN and HCO + lines commonly used as dense gas tracers in extragalactic studies and to test the possible universality of the star-formation efficiency in dense gas (≳10 4 cm -3 ), SFE dense . Methods. We conducted wide-field mapping of the Aquila, Ophiuchus, and Orion B clouds at ~0.04 pc resolution in the J = 1 − 0 transition of HCN, HCO + , and their isotopomers. For each cloud, we derived a reference estimate of the dense gas mass M Herschel A V > 8 , as well as the strength of the local far-ultraviolet (FUV) radiation field, using Herschel Gould Belt survey data products, and estimated the star-formation rate from direct counting of the number of Spitzer young stellar objects. Results. The H 13 CO + (1–0) and H 13 CN(1–0) lines were observed to be good tracers of the dense star-forming filaments detected with Herschel . Comparing the luminosities L HCN and L HCO + measured in the HCN and HCO + lines with the reference masses M Herschel A V > 8 , the empirical conversion factors α Herschel − HCN (= M Herschel A V > 8 / L HCN ) and α Herschel − HCO + (= M Herschel A V > 8 / L HCO + ) were found to be significantly anti-correlated with the local FUV strength. In agreement with a recent independent study of Orion B by Pety et al., the HCN and HCO + lines were found to trace gas down to A V ≳ 2. As a result, published extragalactic HCN studies must be tracing all of the moderate density gas down to n H 2 ≲ 10 3 cm -3 . Estimating the contribution of this moderate density gas from the typical column density probability distribution functions in nearby clouds, we obtained the following G 0 -dependent HCN conversion factor for external galaxies: α Herschel − HCN fit′ = 64 × G 0 -0.34 . Re-estimating the dense gas masses in external galaxies with α Herschel − HCN fit′ ( G 0 ), we found that SFE dense is remarkably constant, with a scatter of less than 1.5 orders of magnitude around 4.5 × 10 -8 yr -1 , over eight orders of magnitude in dense gas mass. Conclusions. Our results confirm that SFE dense of galaxies is quasi-universal on a wide range of scales from ~ 1–10 pc to > 10 kpc. Based on the tight link between star formation and filamentary structure found in Herschel studies of nearby clouds, we argue that SFE dense is primarily set by the “microphysics” of core and star formation along filaments.