THE EVOLUTION OF THE INTERSTELLAR-MEDIUM IN ELLIPTIC GALAXIES .1. THE EARLY WIND PHASE

We investigate the evolution of elliptical galaxies with the goals of understanding the properties of their interstellar medium as well as their effects on the environment. In particular, we study the effects of early supernova-driven winds on the galaxies themselves and on the clusters and compact groups in which these galaxies are the predominant population. Our simulation of the evolution of the interstellar medium in elliptical galaxies includes evolving stellar mass loss rates from Type I and II supernovae, stellar winds, and planetary nebulae. During the first 108-109 yr, the high frequency of Type II supernovae is sufficient to drive early galactic winds in even the most luminous ellipticals. Depending on the stellar birthrate function, the galaxies can eject between 6% to 43% of their initial luminous mass during this early wind phase. The ejected gas is very hot (T ≈ 108 K) and highly enriched with newly synthesized metals with an oxygen to iron ratio approximately twice the solar value. Early winds from elliptical galaxies can exhibit a wide range of iron abundances depending on the birthrate function of the stellar population. We discuss the effects of the hot galactic winds on the energy content and elemental abundance of the interstellar medium. We show that early winds can produce an iron abundance of approximately one-half the solar value in a cluster which presently has an equal mass fraction in gas and stars, if the stellar population formed with a Salpeter initial mass function. These early winds can also heat the intracluster medium to temperatures greater than that produced by gravitational heating alone (i.e., the temperature associated with the velocity dispersion of the galaxies). We also present the results of two numerical simulations concerning the evolution of the interstellar medium in an elliptical galaxy model with a present optical luminosity of LB = 1011 L⊙. The galaxy model consists of a modified King profile for the luminous portion of the galaxy along with an isothermal dark halo which comprises 90% of the total galaxy mass. The stellar population in these models is assumed to form either with an exponentially decreasing star formation rate or instantaneously. After the early galactic wind phase, the models evolve from a pure wind to a pure accretion flow as the rate of Type II supernovae diminishes, and a cooling flow develops which endures until the present time. We allow mass deposition from cooling gas to be distributed over a wide range of radii in constrast with some earlier calculations. The peak X-ray luminosity of the interstellar medium during the early wind phase varies from 1042-1044 ergs s-1, depending on the formation time of the stellar system. At the present time, the X-ray luminosity of the hot coronae in our models is decreasing at a rate given by Lx ∝ t-1,3. We show that the total gas mass of the interstellar medium, the X-ray luminosity, and the X-ray surface brightness after a Hubble time in our models are in good agreement with observations of giant elliptical galaxies.