EVOLUTION OF ELLIPTIC GALAXIES .1. THE MULTIPHASE MODEL

We present a complex model for the evolution of elliptical galaxies in the general framework of the multiphase models for galactic evolution. The elliptical is subdivided into an internal and an external region and their individual but joined evolution is computed in detail. Diffuse gas, molecular clouds, stars, and remnants are taken into account. The star formation rate is therefore assumed to be a two-step process: gas clouds form out of diffuse ps and then stars form out of gas clouds. Cloud-cloud collisions and stimulated processes are considered as the main causes of star formation. The occurrence of winds driven by supernovae (SNs) is taken into account, and the evolution of the system is considered also after the first wind, allowing for further star formation from the restored gas. The evolution of the abundances of 15 elements or isotopes (H, D, He-3, He-4, C-12, C-13, N-14, O-16, Ne-20, Mg-24, Si-28, S-32, Ca-40, Fe-56, and neutron-rich isotopes synthetized from C-12, C-13, N-14, and O-16) is followed with detailed stellar nucleosynthesis; also, stellar lifetimes are taken into account as a consequence of relaxing the instantaneous recycling approximation. A new IMF, constant in space and time, has also been adopted. The gas removal due to the SN explosions depends on the galactic mass and the presence of dark matter; the subsequent wind episodes are crucial to the intergalactic gas enrichment. Good agreement is obtained for current SN rates, star formation rates, and gas masses when compared to the available data.