  Observations show that galaxies follow a mass-metallicity relation over a
wide range of masses. One currently favoured explanation is that less massive
galaxies are less able to retain the gas and stellar ejecta and thus may lose
the freshly produced metals in the form of galactic outflows. Galaxies with a
low current star formation rate have been found to contain star clusters up to
a lower mass limit. Since stars are predominately born in clusters, and less
massive clusters have been found to be less likely to contain very massive
stars, this implies that in environments or at times of low star formation, the
stellar initial mass function does not extend to as high masses as during high
star formation epochs. It is found that the oxygen yield is reduced by a factor
of thirty when the star formation rate is decreased by 3 to 4 orders of
magnitude. With this concept, chemical evolution models for galaxies of a range
of masses are computed and shown to provide an excellent fit to the
mass-metallicity relation derived recently by Tremonti et al. (2004).
Furthermore, the models match the relation between galaxy mass and effective
yield. Thus, the scenario of a variable integrated stellar initial mass
function, which is based on the concept of formation of stars in clusters, may
offer an attractive alternative or partial explanation of the mass-metallicity
relation in galaxies.
