The Evolution of X-Ray Clusters of Galaxies

Considerable progress has been made over the past decade in the study of the evolutionary trends of the population of galaxy clusters in the Universe. In this review we focus on observations in the X-ray band. X-ray surveys with the ROSAT satellite, supplemented by follow-up studies with ASCA and Beppo-SAX, have allowed an assessment of the evolution of the space density of clusters out to z ≈ 1 and the evolution of the physical properties of the intracluster medium out to z ≈ 0.5. With the advent of Chandra and Newton-XMM and their unprecedented sensitivity and angular resolution, these studies have been extended beyond redshift unity and have revealed the complexity of the thermodynamical structure of clusters. The properties of the intracluster gas are significantly affected by nongravitational processes including star formation and Active Galactic Nuclei (AGN) activity. Convincing evidence has emerged for modest evolution of both the bulk of the X-ray cluster population and their thermodynamical properties since redshift unity. Such an observational scenario is consistent with hierarchical models of structure formation in a flat low-density universe with Ωm ≃ 0.3 and σ8 ≃ 0.7–0.8 for the normalization of the power spectrum. Basic methodologies for construction of X-ray–selected cluster samples are reviewed, and implications of cluster evolution for cosmological models are discussed.