Galactic chemical evolution in hierarchical formation models - I. Early-type galaxies in the local Universe

We study the metallicities and abundance ratios of early-type galaxies in cosmological semi-analytic models (SAMs) within the hierarchical galaxy formation paradigm. To achieve this we implemented a detailed galactic chemical evolution model and can now predict abundances of individual elements for the galaxies in the semi-analytic simulations. This is the first time a SAM with feedback from active galactic nuclei has included a chemical evolution prescription that relaxes the instantaneous recycling approximation. We find that the new models are able to reproduce the observed mass-metallicity (M(star)-[ Z/H]) relation and, for the first time in a SAM, we reproduce the observed positive slope of the mass-abundance ratio (M(star)-[alpha/Fe]) relation. Our results indicate that in order to simultaneously match these observations of early-type galaxies, the use of both a very mildly top-heavy initial mass function (i.e. with a slope of x = 1.15 as opposed to a standard x = 1.3), and a lower fraction of binaries that explode as Type Ia supernovae (SNe Ia) appears to be required. We also examine the rate of SN explosions in the simulated galaxies. In early-type (non-star-forming) galaxies, our predictions are also consistent with the observed SNe rates. However, in star-forming galaxies, a higher fraction of SN Ia binaries than in our preferred model is required to match the data. If, however, we deviate from the classical model and introduce a population of SNe Ia with very short delay times, our models simultaneously produce a good match to the observed metallicities, abundance ratios and SN rates.