A fundamental relation between the metallicity, gas content, and stellar
  mass of local galaxies
  Recent results have suggested that the well known mass-metallicity relation
has a strong dependence on the star formation rate, to the extent that a three
dimensional `fundamental metallicity relation' exists which links the three
parameters with minimal scatter. In this work, we use a sample of 4253 local
galaxies observed in atomic hydrogen from the ALFALFA survey to demonstrate,
for the first time, that a similar fundamental relation (the HI-FMR) also
exists between stellar mass, gas-phase metallicity, and HI mass. This latter
relation is likely more fundamental, driving the relation between metallicity,
SFR and mass. At intermediate masses, the behaviour of the gas fundamental
metallicity relation is very similar to that expressed via the star formation
rate. However, we find that the dependence of metallicity on HI content
persists to the highest stellar masses, in contrast to the `saturation' of
metallicity with SFR. It is interesting to note that the dispersion of the
relation is very low at intermediate stellar masses (9< log(M*/Msun) <11),
suggesting that in this range galaxies evolve smoothy, in an equilibrium
between gas inflow, outflow and star formation. At high and low stellar masses,
the scatter of the relation is significantly higher, suggesting that merging
events and/or stochastic accretion and star formation may drive galaxies
outside the relation. We also assemble a sample of galaxies observed in CO.
However, due to a small sample size, strong selection bias, and the influence
of a metallicity-dependent CO/H2 conversion factor, the data are insufficient
to test any influence of molecular gas on metallicity.
