The metal enrichment of elliptical galaxies in hierarchical galaxy formation models

We investigate the metal enrichment of elliptical galaxies in the framework of hierarchical models of galaxy formation. The semi-analytical model we use in this Letter, which has been used to study the metal enrichment of the intracluster medium (ICM) by Nagashima et al., includes the effects of flows of gas and metals both into and out of galaxies and the processes of metal enrichment due to both type Ia and type II supernovae. We adopt a solar neighbourhood IMF for star formation in discs, but consider models in which starbursts have either a solar neighbourhood IMF or a top-heavy IMF. We find that the α-element abundance in ellipticals is consistent with observed values only if the top-heavy IMF is used. This result is consistent with our previous study on the metal enrichment of the ICM. We also discuss the abundance ratio of α elements to iron as a function of velocity dispersion and metallicity. Models with a top-heavy IMF in starbursts seem promising for explaining the observedα/Fe ratios, but some discrepancies with observations remain.