Evolution of Elliptical Galaxies in a Hierarchical Universe

In hierarchical models of galaxy formation, ellipticals form from the merging of disk galaxies drawn together by gravity as their surrounding dark halos coalesce. Using semi-analytic techniques, we are able to follow the merging, star formation and chemical enrichment histories of galaxies for a given choice of cosmological parameters. We calculate the ages and metallicities of elliptical galaxies in a standard CDM cosmology and show that we can reproduce the observed correlation between magnitude and colour, and the scatter about this relation. We also study the evolution of ellipticals to high redshift. In rich clusters, the models predict a substantial population of "evolved" ellipticals at z>1. However, the average number density of bright ellipticals must decrease at high redshift. The rate of decrease is sensitive to cosmological parameters. In a high-density model, the abundance of bright ellipticals declines by factor of 3 by z=1. In a low-density model (Omega=0.3, Lambda=0.7), the abundance drops by 30% at z=1, but is down by a factor 3 at z=2