Mg/Fe ratios in hierarchically-forming ellipticals

In the framework of semi-analytic models for hierarchical galaxy formation, star formation in ellipticals is not truncated but continues to low redshift. I show that, because of this extended star formation history, such models fail to reproduce super-solar Mg/Fe ratios in the global stellar populations. The abundance ratios in the starburst population that forms in the major merger are analysed. It turns out that a considerable flattening of the initial mass function with respect to the Salpeter slope is required in order to achieve overabundant Mg/Fe ratios. The resulting metallicities of the burst population exceed the observed values unless the efficiency of burst star formation is substantially lowered. The abundance features of the global-burst mixture for different proportions and varying IMF slope are discussed.