xGASS: HI fueling of star formation in disk-dominated galaxies
  We introduce a method to estimate the HI mass within the optical radius of
disk galaxies from integrated HI spectra, with an uncertainty of 0.09 dex. We
use these estimates to study how inner HI 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 HI surface density ($\Sigma_{\rm
HI,in}$) and inner HI mass-to-stellar mass ratio. For the massive ($M_*>10^{10}
M_{\odot}$) disk galaxies, higher SFR at a given stellar mass is also related
with higher efficiency of converting inner HI to molecular gas, but no such
correlation is found for the total HI mass. The highest $\Sigma_{\rm HI,in}$
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 HI as an intermediate
step of fueling star formation in disk galaxies.
