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 H-2, Sigma(H2), and the 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 1 kpc. This implies a roughly constant median H-2 consumption time, tau(H2)(Dep) = Sigma(H2)/Sigma(SFR), of similar to 2.35 Gyr (including heavy elements) across our sample. At 1 kpc resolution, there is only a weak correlation between Sigma(H2) and tau(H2)(Dep) over the range Sigma(H2) approximate to 5-100 M-circle dot 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.