Cluster Surveys

I review some of the current efforts to create unbiased samples of galaxy clusters. Readers are referred elsewhere for general wide area sky surveys and redshift surveys, and for Sunyaev-Zeldovich, radio, infrared and submm surveys, some of which were not designed to search primarily for clusters. My focus will be on optical and X-ray samples and on high redshift clusters. Clusters of galaxies are the largest virialized, bound systems known. Their study yields a wealth of information on structure in the Universe over a large range of length scales and on the evolution of that structure over a long time span. Individual clusters are the ideal laboratory for multiwavelength studies of many physical processes involving various constituents of the Universe (galaxies, intracluster hot gas, magnetic fields and relativistic particles) and the effect of the environment on their evolution. On larger scales, samples of clusters are very useful tools to understand the formation and evolution of large-scale structure through the study of the average statistical properties of clusters as a function of redshift. Since clusters are relatively luminous at many wavelengths, they may be more easily traced to very high redshift (even though there are not many clusters known beyond z ≥ 1) and thus they represent a very important observational tool for cosmologists. In particular, they can be used to constrain cosmological parameters. All this explains the past and present efforts made in cluster surveys to build very large cluster samples and to push the detection of clusters to higher and higher redshift. In this respect the search for clusters has greatly benefitted from new technologies and observational capabilities. Faint spectroscopic data from Keck and VLT, deep optical and near-infrared imaging in space (HST) and from the ground, observations of the intracluster medium with ROSAT and ASCA, enhanced imaging and spectroscopic capabilities of Chandra and XMM-Newton, have considerably added to our knowledge of these objects.