Galaxy Clusters Around Quasars

Abstract

A number of systematic surveys of fields around quasars have been carried out in the past decade to study their global environments. This paper summarizes some of the results, especially those pertaining to galaxy clusters around quasars. Quasars are found in environments significantly richer than those of average galaxies. However, at low redshift (z 〈 0.4), few, if any, are located in rich clusters. Nevertheless, a significant fraction of optically bright radio-loud quasars at z ~ 0.6 are found to be at or near the centers of galaxy clusters of Abell class 1 or richer. This can be understood if there is a much more rapid evolution of quasars in rich clusters compared with those in poorer environments. The evolution interpretation is confirmed by a survey of faint quasars in which lower luminosity quasars at lower redshift are also found in rich clusters. Possible candidates for the end products of this rapid evolution of quasars in rich clusters are radio galaxies in rich clusters at low redshift. By combining data from quasars and low-redshift radio galaxies, it is shown that they can both be consistently described by a single luminosity function and evolution law. Possible explanations for the more rapid evolution of quasar activity in clwiters are discussed. Preliminary results from multi-object spectroscopy of galaxies around quasars are described. In general, clusters hosting bright quasars appear to have relatively low velocity dispersions, suggesting that the dynamical state of the cluster plays a role in the evolution of these quasars. Furthermore, interactions and mergers are important mechanisms in the triggering and maintenance of quasar activity.