MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES

We compare molecular gas traced by (CO)-C-12 (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 Sigma(mol) and Sigma(SFR) but also find important second-order systematic variations in the apparent molecular gas depletion time, tau(mol)(dep) = Sigma(mol)/Sigma(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 alpha(CO) equivalent to the Milky Way value, our data yield a molecular gas depletion time, tau(mol)(dep) = Sigma(mol)/Sigma(SFR) approximate to 2.2 Gyr with 0.3 dex 1 sigma 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, Sigma(SFR) proportional to Sigma(N)(mol). We find N = 1 +/- 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 tau(mol)(dep) and other local and global quantities. The strongest of these are a decreased tau(mol)(dep) in low-mass, low-metallicity galaxies and a correlation of the kpc-scale tau(mol)(dep) with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H-2 conversion factor (alpha(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 tau(mol)(dep) trends. After applying a D/G-dependent alpha(CO), some weak correlations between tau(mol)(dep) and local conditions persist. In particular, we observe lower tau(dep)(mol) 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 tau(dep) does not appear bimodal in galaxy centers, tau(dep) does appear multivalued at fixed Sigma(H2), supporting the idea of disk and starburst modes driven by other environmental parameters.