A CONSTANT MOLECULAR GAS DEPLETION TIME IN NEARBY DISK GALAXIES

Abstract

We combine new sensitive, wide-field CO data from the HERACLES survey with\nultraviolet and infrared data from GALEX and Spitzer to compare the surface\ndensities of H2, Sigma_H2, and recent star formation rate, Sigma_SFR, over many\nthousands of positions in 30 nearby disk galaxies. We more than quadruple the\nsize of the galaxy sample compared to previous work and include targets with a\nwide range of galaxy properties. Even though the disk galaxies in this study\nspan a wide range of properties, we find a strong and approximately linear\ncorrelation between Sigma_SFR and Sigma_H2 at our common resolution of 1kpc.\nThis implies a roughly constant median H2 consumption time, tau_H2 = Sigma_H2 /\nSigma_SFR, of ~2.35Gyr (including heavy elements) across our sample. At 1kpc\nresolution, there is only a weak correlation between Sigma_H2 and tau_H2 over\nthe range Sigma_H2~5-100M_sun/pc^2, which is probed by our data. We compile a\nbroad set of literature measurements that have been obtained using a variety of\nstar formation tracers, sampling schemes and physical scales and show that\noverall, these data yield almost exactly the same results, although with more\nscatter. We interpret these results as strong, albeit indirect evidence that\nstar formation proceeds in a uniform way in giant molecular clouds in the disks\nof spiral galaxies.\n