X-ray Observations of Groups and Clusters - A Relic of the High Redshift Universe

Abstract

I review the evidence from clusters and groups of galaxies for `relic' evidence of the high redshift universe. Contrary to the received wisdom, clusters are old. Their x-ray emitting intergalactic medium in massive clusters is a reservoir of metal and energy injection from the main epoch of star formation. There are strong indications that non-gravitational energy was extremely important in the formation of low-mass clusters and groups, implying that gravity did not dominate the process on all scales below that of massive, rich clusters. The data indicate that most of the metals in the universe are not in stars but in the hot gas and thus the use of stars to trace metal formation and evolution misses most of the processed material. The group data shows that galaxies are very poor tracers of mass on the scale of 10^13 M_Sun and since groups are the `average' place in the universe this probably holds for most of the visible universe.