Iron as a Tracer in Galaxy Clusters and Groups

Available X-ray data are collected and organized concerning the iron and gas content of galaxy clusters and groups, together with the optical luminosity, mass, and iron abundance of cluster galaxies. Moving from such a restricted number of cluster parameters, several astrophysical inferences are drawn. These include the evidence for rich clusters having evolved without much baryon exchange with their surroundings and having experienced very similar star formation histories. Groups are more gas-poor as compared with clusters and appear instead to have shed a major fraction of their original cosmic share of baryons, which indicates that galaxy clusters cannot have formed by assembling groups that are similar to the present-day ones. It is argued that this favors low-Ω universes, in which the growth of rich clusters is virtually complete at high redshifts. It is also argued that elemental abundance ratios in clusters are nearly solar, which is consistent with a similar proportion of supernovae of Type Ia and Type II having enriched both the solar neighborhood and clusters as a whole. Much of the iron in clusters appears to reside in the intracluster medium rather than inside galaxies, the precise ratio being a function of the Hubble constant. It appears that the baryon to star conversion in clusters has been nearly as efficient as that currently observed for the universe as a whole. Yet the metallicity of the clusters is ~5 times higher than the global metallicity found for the nearby universe. It is concluded that the intergalactic medium should have a metallicity ~1/3 of the solar value if stellar nucleosynthesis has proceeded in stars within field galaxies with the same efficiency as in stars within clusters of galaxies.