The Universal Relation of Galactic Chemical Evolution: The Origin of the
  Mass-Metallicity Relation
  We examine the mass-metallicity relation for $z\lesssim 1.6$. The
mass-metallicity relation follows a steep slope with a turnover or `knee' at
stellar masses around $10^{10} M_\odot$. At stellar masses higher than the
characteristic turnover mass, the mass-metallicity relation flattens as
metallicities begin to saturate. We show that the redshift evolution of the
mass-metallicity relation depends only on evolution of the characteristic
turnover mass. The relationship between metallicity and the stellar mass
normalized to the characteristic turnover mass is independent of redshift. We
find that the redshift independent slope of the mass-metallicity relation is
set by the slope of the relationship between gas mass and stellar mass. The
turnover in the mass-metallicity relation occurs when the gas-phase oxygen
abundance is high enough that the amount of oxygen locked up in low mass stars
is an appreciable fraction of the amount of oxygen produced by massive stars.
The characteristic turnover mass is the stellar mass where the stellar-to-gas
mass ratio is unity. Numerical modeling suggests that the relationship between
metallicity and stellar-to-gas mass ratio is a redshift independent, universal
relationship followed by all galaxies as they evolve. The mass-metallicity
relation originates from this more fundamental universal relationship between
metallicity and stellar-to-gas mass ratio. We test the validity of this
universal metallicity relation in local galaxies where stellar mass,
metallicity and gas mass measurements are available. The data are consistent
with a universal metallicity relation. We derive an equation for estimating the
hydrogen gas mass from measurements of stellar mass and metallicity valid for
$z\lesssim1.6$ and predict the cosmological evolution of galactic gas masses.
