Abundance ratios in hierarchical galaxy formation

The chemical enrichment and stellar abundance ratios of galaxies that form in a hierarchical clustering scheme are calculated. For this purpose, I adopt the star formation histories (SFH) as they are delivered by semi-analytic models by Kauffmann. It turns out that the average SFH of cluster ellipticals does not yield globally a-enhanced stellar populations. The starburst that occurs when the elliptical forms in the major merger therefore plays a crucial role in producing a-enhancement. Only under the assumption that the initial mass function is signi®cantly ¯attened with respect to the Salpeter value during the burst, can a Mg/Fe overabundant population be obtained. In particular, for the interpretations of radial gradients in metallicity and a-enhancement, the mixing of global and burst populations are of great importance. The model predicts bright ®eld galaxies to be less a-enhanced than their counterparts in clusters.