A chemical evolution model is combined with a fully hydrodynamical code to follow the evolution of elliptical galaxies from the protogalaxy stage. In this way, the single-zone assumption, usual in chemical evolution models, is dropped. This allows the investigation of radial metallicity gradients, and, in particular the formation of the high-metallicity core in ellipticals. The star formation rate and the subsequent supernova heating regulate the episodes of wind, outflow, and cooling flow, thus affecting the recycling of the gas and the chemical enrichment of the intergalactic medium.

Introduction

In this work, a chemical evolution model is combined with a hydrodynamical model to follow the evolution of elliptical galaxies from the protogalactic stage. One motivation for the present study comes from the starburst model for QSOs (Terlevich & Boyle 1993 and references therein). In this model, the QSOs are the young cores of massive ellipticals forming most of the dominant metal-rich population in a short starburst. Since QSOs are seen up to redshifts of z ∼ 5, the suprasolar metallicities required by this models should be reached by ∼ 1 Gyr since the epoch of galaxy formation. This evolutionary time scale is an important constraint in chemical enrichment models. Hamann & Ferland (1992) have used one-zone chemical evolution models to investigate the chemical history of QSOs with results consistent with the starburst model of QSOs.