Abstract details

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 or magnitude. With this concept, chemical evolution models for galaxies are computed and shown to provide an excellent fit to the mass--metallicity relation derived recently by Tremonti et al. (2004). Thus, the scenario of a variable integrated stellar IMF, which is based on the concept of formation of stars in clusters, offer an attractive alternative explanation of the mass--metallicity relation.