The evolution of the mass-metallicity relation in galaxies of different
  morphological types
  By means of chemical evolution models for ellipticals, spirals and irregular
galaxies, we aim at investigating the physical meaning and the redshift
evolution of the mass-metallicity relation as well as how this relation is
connected with galaxy morphology. {abridged} We assume that galaxy morphologies
do not change with cosmic time. We present a method to account for a spread in
the epochs of galaxy formation and to refine the galactic mass grid. (abridged)
We compare our predictions to observational results obtained for galaxies
between redshifts 0.07 and 3.5. We reproduce the mass-metallicity (MZ) relation
mainly by means of an increasing efficiency of star formation with mass in
galaxies of all morphological types, without any need to invokegalactic
outflows favoring the loss of metals in the less massive galaxies. Our
predictions can help constraining the slope and the zero point of the observed
local MZ relation, both affected by uncertainties related to the use of
different metallicity calibrations. We show how, by considering the MZ, the O/H
vs star formation rate (SFR), and the SFR vs galactic mass diagrams at various
redshifts, it is possible to constrain the morphology of the galaxies producing
these relations. Our results indicate that the galaxies observed at z=3.5
should be mainly proto-ellipticals, whereas at z=2.2 the observed galaxies
consist of a morphological mix of proto-spirals and proto-ellipticals. At lower
redshifts, the observed MZ relation is well reproduced by considering both
spirals and irregulars. (abridged)
