The dominant form of baryons in galaxy clusters is X-ray emitting plasma (hot gas). The central regions of these clusters are among the most interesting and physically active areas in the Universe. Many such clusters harbor a central peak in X-ray surface brightness, with a central cooling time for the intracluster gas which is less than the age of the cluster.

Without additional heating, the cooled gas would flow into the center in the form of a "cooling flow". The gas should therefore continue to cool to very low temperatures, but X-ray observations fail to find enough cooled gas to be consistent with this picture. Often, the clusters host central dominant galaxies which are also strong radio sources. It is now widely believed that the central radio sources are interacting with the surrounding X-ray emitting plasma and are heating the surrounding gas.

This false color image shows the X-ray emitting plasma of a moderate cooling flow cluster called Abell 2626, obtained by the XMM-Newton Observatory. The redder regions are associated with brighter emission in X-rays. The image region shown above spans an area of roughly 1.5 Mpc by 1.5 Mpc at the cluster's estimated distance of 253 Mpc. The global morphology of Abell 2626 appears to be roughly azimuthally symmetric, but in the very central region (the inner ~100 kpc), there is evidence that relativistic radio plasma is interacting with the X-ray emitting thermal plasma. The interaction has been studied by Ka-Wah Wong and Craig Sarazin using both XMM-Newton and Chandra observations.