Evolution of hot gas and dark halos in group-dominant elliptical galaxies:Influence of cosmic inflow

Hot interstellar gas in elliptical galaxies has two sources: mass lost from evolving stars and a much older component that accompanied galaxy formatio n or arrived subsequently by secondary cosmic infall toward the galaxy grou p containing the elliptical. We present here an approximate but comprehensi ve study of the dynamical evolution of the hot gas in massive elliptical ga laxies born into a simple flat universe. Baryonic and dark matter are both conserved. We use NGC 4472 as a prototypical massive elliptical having a we ll-observed hot interstellar medium. We allow for star formation in a simpl e single burst using a Salpeter initial mass function but treat the gasdyna mics in detail. The galaxy has a de Vaucouleurs stellar core and a Navarro- Frenk-White dark halo surrounded by inflowing cosmic matter. Using rather s tandard assumptions and parameters, we are able to successfully reproduce t he gas-density and temperature distributions-n(r) and T(r)-in the hot inter stellar gas determined from recent X-ray observations. Our model is sensiti ve to the baryon fraction of the universe, the Type II supernova energy rel eased per unit stellar mass, and the time of galaxy formation. However, the re is some degeneracy; as each of these parameters is varied, the effect on model fits to n(r) and T(r) is similar. Nevertheless, secondary inflow of cosmic gas is essential for successful fits to n(r) and T(r). Some gas is e xpelled from the stellar galactic core at early times when the Type II supe rnova energy is released. As a result, the present day baryonic fraction ha s a deep minimum in the outer galactic halo. Interstellar gas that cooled s ince the time of maximum star formation cannot all have collected at the ga lactic center but must be spatially dispersed; otherwise both gas temperatu res and stellar dispersions in the galactic center would be larger than tho se observed. Finally, when relatively gas-rich, X-ray luminous models are s patially truncated at early times, simulating tidal events that may have oc curred during galaxy group dynamics, the current locus of truncated models lies just along the L-X-X-ray size correlation among well-observed elliptic als. This is another striking confirmation of our model of elliptical evolu tion.