Explaining the scatter in the galaxy mass-metallicity relation with gas
  flows
  The physical origin of the scatter in the relation between galaxy stellar
mass and the metallicity of the interstellar medium, i.e. the Mass-Metallicity
Relation (MZR), reflects the relative importance of key processes in galaxy
evolution. The \eagle cosmological hydrodynamical simulation is used to
investigate the correlations between the residuals of the MZR and the residuals
of the relations between stellar mass and, respectively, specific inflow,
outflow and star formation rate as well as the gas fraction for central
galaxies. At low redshift, all these residuals are found to be anti-correlated
with the residuals of the MZR for $M_\star/\mathrm{M}_\odot \lesssim 10^{10}$.
The correlations between the residuals of the MZR and the residuals of the
other relations with mass are interrelated, but we find that gas fraction,
specific inflow rate and specific outflow rate all have at least some
independent influence on the scatter of the MZR. We find that, while for
$M_\star/\mathrm{M}_\odot > 10^{10.4}$ the specific mass of the nuclear black
hole is most important, for $M_\star/\mathrm{M}_\odot \lesssim 10^{10.3}$ gas
fraction and specific inflow rate are the variables that correlate most
strongly with the MZR scatter. The timescales involved in the residual
correlations and the time that galaxies stay above the MZR are revealed to be a
few Gyr. However, most galaxies that are below the MZR at $z=0$ have been below
the MZR throughout their lifetimes.
