Radio Observations of Galaxy Clusters: Connection to Cluster Mergers
  The main component of the intracluster medium (ICM) in clusters of galaxies is represented by the X-ray emitting thermal plasma. In addition, the presence of relativistic electrons and large-scale magnetic fields in a fraction of galaxy clusters is demonstrated by the detection of large-scale synchrotron radio sources, which have no optical counterpart and no obvious connection to the cluster galaxies. Observational results provide evidence that these phenomena are related to cluster merger activity, which supplies the energy for the reacceleration of the radio emitting particles. The investigation of the halo-merger connection is of great importance to the knowledge of the formation and evolution of clusters of galaxies. 
L. Feretti