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A Constant Molecular Gas Depletion Time in Nearby Disk Galaxies

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