HI gas content of SDSS galaxies revealed by ALFALFA: implications for
  the mass-metallicity relation and the environmental dependence of HI in the
  local Universe
  The neutral hydrogen~(HI) gas is an important barometer of recent star
formation and metal enrichment activities in galaxies. I develop a novel
statistical method for predicting the HI-to-stellar mass ratio $f_{gas}$ of
galaxies from their stellar mass and optical colour, and apply it to a
volume-limited galaxy sample jointly observed by the Sloan Digital Sky Survey
and the Arecibo Legacy Fast ALFA survey. I eliminate the impact of the
Malmquist bias against HI-deficient systems on the $f_{gas}$ predictor by
properly accounting for the HI detection probability of each galaxy in the
analysis. The best-fitting $f_{gas}$ predictor, with an estimated scatter of
$0.272$ dex, provides excellent description to the observed HI mass function.
After defining an HI excess parameter as the deviation of the observed
$f_{gas}$ from the expected value, I confirm that there exists a strong
secondary dependence of the mass-metallicity relation on HI excess. By further
examining the 2D metallicity distribution on the specific star formation rate
vs. HI excess plane, I show that the metallicity dependence on HI is likely
more fundamental than that on specific star formation rate. In addition, I find
that the environmental dependence of HI in the local Universe can be
effectively described by the cross-correlation coefficient between HI excess
and the red galaxy overdensity $\rho_{cc}{=}-0.18$. This weak anti-correlation
also successfully explains the observed dependence of HI clustering on
$f_{gas}$. My method provides a useful framework for learning HI gas evolution
from the synergy between future HI and optical galaxy surveys.
