Radio haloes in galaxy clusters: what do they tell us about the cluster's magnetic field?

Our new radio observations of a complete, X-ray selected sample of clusters of galaxies suggest that diffuse radio haloes exist in all clusters, not just those undergoing a major merger. Some of our detected haloes are in clusters with strong cooling cores, which are thought to be dynamically relaxed. We infer that the plasma in every cluster contains relativistic particles and magnetic fields, whether or not the cluster has recently undergone a major merger. We do not know, however, what energizes the plasma in relaxed clusters. The answer could be mesoscale fluctuations in the underlying dark matter, or energy input from active galaxies, or a mixture of both. In addition, Faraday rotation from the plasma seems to require a magnetic field significantly stronger than that inferred from simple analyses of the radio haloes and/or hard X-ray emission. We suspect this apparent problem comes from our incomplete understanding of magnetic structures and relativistic plasmas in the cluster; we suggest an alternative way to interpret radio and hard X-ray data. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.