Gas accretion regulates the scatter of the mass-metallicity relation
  In this paper, we take advantage of the GAlaxy Evolution and Assembly (GAEA)
semi-analytic model to analyse the origin of secondary dependencies in the
local galaxy mass - gas metallicity relation. Our model reproduces quite well
the trends observed in the local Universe as a function of galaxy star
formation rate and different gas-mass phases. We show that the cold gas content
(whose largest fraction is represented by the atomic gas phase) can be
considered as the third parameter governing the scatter of the predicted
mass-metallicity relation, in agreement with the most recent observational
measurements. The trends can be explained with fluctuations of the gas
accretion rates: a decrease of the gas supply leads to an increase of the gas
metallicity due to star formation, while an increase of the available cold gas
leads to a metallicity depletion. We demonstrate that the former process is
responsible for offsets above the mass-metallicity relation, while the latter
is responsible for deviations below the mass-metallicity relation. In low and
intermediate mass galaxies, these negative offsets are primarily determined by
late gas cooling dominated by material that has been previously ejected due to
stellar feedback.
