Galaxy metallicities depend primarily on stellar mass and molecular gas
  mass
  In this work we present an analysis of the behaviour of galaxies in a
four-dimensional parameter space defined by stellar mass, metallicity, star
formation rate, and molecular gas mass. We analyse a combined sample of 227
galaxies, which draws from a number of surveys across the redshift range 0 < z
< 2 (> 90% of the sample at z~0), and covers > 3 decades in stellar mass.Using
Principle Component Analysis, we demonstrate that galaxies in our sample lie on
a 2-dimensional plane within this 4D parameter space, indicative of galaxies
that exist in an equilibrium between gas inflow and outflow. Furthermore, we
find that the metallicity of galaxies depends only on stellar mass and
molecular gas mass. In other words, gas-phase metallicity has a negligible
dependence on star formation rate, once the correlated effect of molecular gas
content is accounted for. The well-known `fundamental metallicity relation',
which describes a close and tight relationship between metallicity and SFR (at
fixed stellar mass) is therefore entirely a by-product of the underlying
physical relationship with molecular gas mass (via the Schmidt-Kennicutt
relation).
