The Evolving Stellar Content of Galaxies and the X-Ray Evolution of Elliptical Galaxies

Abstract

Some of the several rôles played by the evolving stellar component of galaxies are introduced, and the ultimate root of many observational correlations among global galaxy properties is indicated in the current stellar death rate. As a concrete example, the case of the stellar mass loss in elliptical galaxies is examined in detail. The material lost by stars is heated to high temperatures by thermalization to the stellar velocity dispersion, while supernovae of type I provide the major heat source. Hydrodynamical models for the resulting gas flows enlight the important rôle played by the cosmological evolution of the rate of type I supernovae, which together with the shape and depth of the galactic potential well (sensitive to the amount of dark matter) concur in determining the nature of the flows. The flows can then experience up to three consecutive evolutionary phases: a supersonic wind, a subsonic outflow, and an inflow phase. The available X-ray data favor subsonic outflows as the prevalent phase among present epoch ellipticals.