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.
