Magnetic fields of microGauss strength are present in galaxy clusters. Cluster mergers input a great deal of energy, some of which tangles magnetic fields, and some of which accelerates relativistic particles. For a short period after a major merger, the cluster is seen as a radio halo. The radio emission rapidly fades as the relativistic electrons lose energy, but the magnetic fields dissipate extremely slowly and are built up over the lifetime of the cluster.

The ultimate origin of intracluster magnetic fields is most likely left-over radio sources. A single powerful radio source contains sufficient energy to magnetize the core of a galaxy cluster. A rough integration over all the sources in the history of cluster showed that most clusters have accumulated magnetic energy equivalent to several powerful radio sources in their lifetimes.

Peter Tribble,