xGASS: H i Fueling of Star Formation in Disk-dominated Galaxies

We introduce a method to estimate the H i mass within the optical radius of disk galaxies from integrated H i spectra, with an uncertainty of 0.09 dex. We use these estimates to study how inner H i fuels star formation in late-type disk galaxies. We find that star formation rate (SFR) at a given stellar mass (M *) is well correlated with the inner H i surface density (${{\rm{\Sigma }}}_{{\rm{H}}{\rm{I}},\mathrm{in}}$) and inner H i mass-to-stellar mass ratio. For the massive (M * > 1010 M ⊙) disk galaxies, higher SFR at a given stellar mass is also related to higher efficiency of converting inner H i to molecular gas, but no such correlation is found for the total H i mass. The highest ${{\rm{\Sigma }}}_{{\rm{H}}{\rm{I}},\mathrm{in}}$ and the fastest depletion of the total neutral gas within the optical disks are found in the most compact and star-forming disk galaxies at a given stellar mass. These results highlight the important role of inner H i as an intermediate step of fueling star formation in disk galaxies.