Hot plasma in clusters of galaxies, the largest objects in the universe

Clusters of galaxies are the largest organized structures in the Universe. They are important cosmological probes, since they are large enough to contain a fair sample of the materials in the Universe, but small enough to have achieved dynamical equilibrium. Clusters were first discovered as concentrations of hundreds of bright galaxies in a region about 3 megaparsecs (10 million light years) across. However, the dominant observed form of matter in clusters is hot, diffuse intergalactic gas. This intracluster plasma has typical temperatures of T ~ 7e7 K, and typical electron densities of n_e ~ 10^{-3} cm^{-3}. This intracluster plasma mainly emits X-rays, and typical cluster X-ray luminosities are L_X ~ 10^{43} - 10^{45} erg/s. The basic properties of and physical processes in the intracluster plasma will be reviewed. Important observational constraints on plasma processes in these systems will be discussed. Recent X-ray observations of clusters of galaxies with the orbiting Chandra X-ray Observatory will be highlighted.