Entropy Evolution in Galaxy Groups and Clusters; A Comparison of External and Internal Heating

Abstract

X-ray observations of hot gas in galaxy groups indicate higher entropies than can be achieved in the accretion shocks as it fell into the dark halos. It has been proposed that this entropy excess results from some universal external heating process in the early universe - AGN, population III stars, etc. - that occurred before most of the gas entered the dark halos. Alternatively, the hot gas in groups may be heated internally by Type II supernovae when the galactic stars formed. We investigate here the gas-dynamical consequences of various amounts of external, high redshift heating and compare with internal supernova heating. Radiative losses and distributed mass dropout are included. We find that externally heated flows are unsatisfactory; when the heating is high enough to bring the X-ray luminosities into agreement with observations, the gas entropy is too high. Flows internally heated by Type II supernovae are preferred. Our results are insensitive to the epoch and spatial distribution of the supernova heating process. However, with a Salpeter IMF a large fraction of the energy produced by supernovae must heat the hot gas. Alternatively, only about 20 percent of the Type II supernova energy would be required to heat the gas if the initial mass function is flatter than Salpeter, as suggested by stellar mass to light ratios.