The role of atomic hydrogen in regulating the scatter of the
  mass-metallicity relation
  In this paper, we stack neutral atomic hydrogen (HI) spectra for 9,720 star
forming galaxies along the mass-metallicity relation. The sample is selected
according to stellar mass (10$^9 \leq$ M$_{\star}$/M$_{\odot}\leq$10$^{11}$)
and redshift ($0.02 \leq z \leq 0.05$) from the overlap of the Sloan Digital
Sky Survey and Arecibo Legacy Fast ALFA survey. We confirm and quantify the
strong anti-correlation between HI mass and gas-phase metallicity at fixed
stellar mass. Furthermore, we show for the first time that the relationship
between gas content and metallicity is consistent between different metallicity
estimators, contrary to the weaker trends found with star formation which are
known to depend on the observational techniques used to derive oxygen
abundances and star formation rates. When interpreted in the context of
theoretical work, this result supports a scenario where galaxies exist in an
evolving equilibrium between gas, metallicity and star formation. The fact that
deviations from this equilibrium are most strongly correlated with gas mass
suggests that the scatter in the mass-metallicity relation is primarily driven
by fluctuations in gas accretion.
