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, ΣH2, and the recent star formation rate, ΣSFR, 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 ΣSFR and ΣH2 at our common resolution of 1 kpc. This implies a roughly constant median H2 consumption time, τH2 Dep = ΣH2/ΣSFR, of ~2.35 Gyr (including heavy elements) across our sample. At 1 kpc resolution, there is only a weak correlation between ΣH2 and τH2 Dep over the range ΣH2 ≈ 5-100 M 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.