Chemical enrichment and the origin of the colour—magnitude relation of elliptical galaxies in a hierarchical merger model

Abstract

In this paper, we present a model of the formation and chemical enrichment of elliptical galaxies that differs from the conventional picture in two ways: 1)Ellipticals do not form in a single monolithic collapse and burst of star formation at high redshift. Instead, most of their stars form at modest rates in disk galaxies, which then merge to form the ellipticals. 2)Galaxies do not undergo closed-box chemical evolution. Instead, metals can be transferred between the stars, cold gas and the hot gas halos of the galaxies. It is assumed that metals are ejected out of disk galaxies during supernova explosions and these metals enter the hot gas component. The fact that metals are more easily ejected from small galaxies leads to the establishment of a mass-metallicity relation for the disk systems. Big ellipticals are more metal rich because they are formed from the mergers of bigger disks. We use semi-analytic techniques to follow the formation, evolution, and chemical enrichment of cluster ellipticals in a merging hierarchy of dark matter halos. The inclusion of the new metallicity-dependent spectral synthesis models of Bruzual & Charlot allow us to compute the colours, line indices and mass-to- light ratios of these galaxies. If feedback is assumed to be efficient, we are able to reproduce the slope and scatter of the colour-magnitude and the Mg2-sigma relations. We also study the evolution of these relations to high redshift. We show that the luminosity-metallicity relation remains fixed, but the mean stellar ages of the galaxies scale with the age of the Universe. Finally, we study the enrichment history of the intracluster gas. We find that 80% of the metals were ejected by low-mass galaxies at redshifts greater than 1. The metallicity of the ICM thus evolves very little out to z > 1.