Evolution of Hot Gas and Dark Halos in Group‐dominant Elliptical Galaxies: Influence of Cosmic Inflow

Abstract

Hot interstellar gas in elliptical galaxies has two sources: mass lost from evolving stars and a much older component that accompanied galaxy formation or arrived subsequently by secondary cosmic infall toward the galaxy group containing the elliptical. We present here an approximate but comprehensive study of the dynamical evolution of the hot gas in massive elliptical galaxies born into a simple flat universe. Baryonic and dark matter are both conserved. We use NGC 4472 as a prototypical massive elliptical having a well-observed hot interstellar medium. We allow for star formation in a simple single burst using a Salpeter initial mass function but treat the gasdynamics in detail. The galaxy has a de Vaucouleurs stellar core and a Navarro-Frenk-White dark halo surrounded by inflowing cosmic matter. Using rather standard assumptions and parameters, we are able to successfully reproduce the gas-density and temperature distributions—n(r) and T(r)—in the hot interstellar gas determined from recent X-ray observations. Our model is sensitive to the baryon fraction of the universe, the Type II supernova energy released per unit stellar mass, and the time of galaxy formation. However, there 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 expelled from the stellar galactic core at early times when the Type II supernova energy is released. As a result, the present day baryonic fraction has a deep minimum in the outer galactic halo. Interstellar gas that cooled since the time of maximum star formation cannot all have collected at the galactic center but must be spatially dispersed; otherwise both gas temperatures and stellar dispersions in the galactic center would be larger than those observed. Finally, when relatively gas-rich, X-ray luminous models are spatially truncated at early times, simulating tidal events that may have occurred during galaxy group dynamics, the current locus of truncated models lies just along the L-X-ray size correlation among well-observed ellipticals. This is another striking confirmation of our model of elliptical evolution.