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.