Cool Gas in Clusters of Galaxies

Early X-ray observations suggested that the intracluster medium cools and condenses at the centers of clusters, leading to a cooling flow of plasma in the cluster core. The increased incidence of emission-line nebulosity, excess blue light, AGN activity, and molecular gas in the cores of clusters with short central cooling times seemed to support this idea. However, high-resolution spectroscopic observations from XMM-Newton and Chandra have conclusively ruled out simple, steady cooling flow models. We review the history of this subject, the current status of X-ray observations, and some recent models that have been proposed to explain why the core gas does not simply cool and condense. 1.1 A Census of Cool Gas Clusters of galaxies have very deep potential wells with virial velocities equivalent to temperatures of 107 − 108 K. Gravitationally driven processes like accretion shocks and adiabatic compression should therefore heat gas accumulating within a cluster to Xray emitting temperatures. Spectroscopic X-ray observations show that most of a cluster’s gas is indeed near the virial temperature Tvir = μmpσ 1D/k, equivalent to 7.1× 10 7σ2 1000 K or 6.2σ2 1000 keV, where σ1000 is the line-of-sight velocity dispersion in units of 1000kms −1