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, email@example.com