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_* > 10¹⁰M_⊙) 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.