Molecular gas and star formation in nearby disk galaxies

Abstract

We compare molecular gas traced by 12CO (2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between Σmol and ΣSFR but also find important second-order systematic variations in the apparent molecular gas depletion time, τmoldep = Σmol/Σ SFR. At the 1 kpc common resolution of HERACLES, CO emission correlates closely with many tracers of the recent SFR. Weighting each line of sight equally, using a fixed αCO equivalent to the Milky Way value, our data yield a molecular gas depletion time, τmoldep = Σmol/ΣSFR ≈ 2.2 Gyr with 0.3 dex lσ scatter, in very good agreement with recent literature data. We apply a forward-modeling approach to constrain the power-law index, N, that relates the SFR surface density and the molecular gas surface density, ΣSFR ∝ σNmol;. We find N = l ± 0.15 for our full data set with some scatter from galaxy to galaxy. This also agrees with recent work, but we caution that a power-law treatment oversimplifies the topic given that we observe correlations between τmol;dep and other local and global quantities. The strongest of these are a decreased τmol;dep in low-mass, low-metallicity galaxies and a correlation of the kpc-scale τmoldep with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H2 conversion factor (∝CO) that depends on dust shielding, and thus D/G, in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes this the most conservative explanation of the strongest observed τmoldep trends. After applying a D/G-dependent ∝CO, some weak correlations between τmoldep and local conditions persist. In particular, we observe lower τmoldep and enhanced CO excitation associated with nuclear gas concentrations in a subset of our targets. These appear to reflect real enhancements in the rate of star formation per unit gas, and although the distribution of τdep does not appear bimodal in galaxy centers, τdep does appear multivalued at fixed ΣH2, supporting the idea of "disk" and "starburst" modes driven by other environmental parameters. © 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.