The baryonic content of galaxies and clusters

Optical and X-ray observations of early-type galaxies, groups, and clusters of galaxies show that a greater fraction of the gravitating mass is observable in rich dusters than in individual galaxies. In elliptical galaxies, only a few percent of the gravitating mass is observed in stars or gas, while in rich clusters, approximately 30% of the mass is contained in hot gas and galaxies. In this Letter, we show that if the recent estimate of the MACHO abundance in our Galaxy is representative of all galaxies, then the ''observable'' content of galaxies, groups, and rich clusters (in the form of MACHOs, stars, and gas) is nearly constant at 50% of the total mass. We also explore the consequences for the gaseous content of groups and clusters if MACHOs are the stellar remnants of an early population of stars. Previous studies have shown that the hot gas in clusters cannot be supplied by the present population of stars in clusters, which implies that the gas is primordial. However, based on the MACHO detection rate in our Galaxy, we find that the stellar progenitors of MACHOs could produce the observed gas mass and abundance of heavy elements in rich clusters. The observed trend of increasing gas mass fraction between groups and clusters then can be explained within the context of hierarchical clustering scenarios.