Mass-metallicity relation of dwarf galaxies and its dependency on time:
  clues from resolved systems and comparison with massive galaxies
  We present a new approach to study the mass-metallicity relation and its
dependency on time. We used the star formation history (SFH) derived from
color-magnitude diagram fitting techniques of a sample of Local Group (LG)
dwarfs to obtain stellar masses, metallicities, and star-formation rates (SFR)
to analyze the mass-metallicity relation as a function of the ages of their
stellar populations. The accurate SFHs allow a time resolution of about 2 Gyr
at the oldest ages for a total redshift range of 0<~z<~3. The mass-metallicity
relation retrieved for the sample of LG dwarfs was compared with a large
dataset of literature data obtained in a wide redshift range. Neither of the
two independent datasets shows a clear evolution of the mass-metallicity
relation slope with redshift. However, when the star-formation rate is added as
an additional parameter in the relation, it shows a dependence on the redshift
in the sense that the coefficient of the mass decreases with increasing
redshift, while the coefficient for the SFR is almost constant with time. This
result suggests an increasing contribution with time of the galaxy stellar mass
to the metalliticy of the stars that formed most recently, but it also shows
that the SFR can play a fundamental role in shaping the mass-metallicity
relation.
