Direct evidence for the existence of magnetic fields in at least some clusters is given by the presence of extended radio halo emission. This radio synchrotron emission requires that both relativistic electrons and magnetic fields be present in the cluster. The large extent of the halo causes a problem, as the lifetime of the relativistic electrons is insufficient to allow them to diffuse from a small number of locations to fill the entire cluster.

This problem is solved if the halos are a transient phenomenon apparent just after a major cluster merger event (Tribble 1993). This explains the rarity of cluster halos and the properties of the clusters they are found in - there is a very close correspondence between halo clusters and clusters that X-ray evidence indicates have recently merged (Edge, Stewart & Fabian 1992; Watt et al. 1992). The energy available in a merger event is very large---of order the thermal energy in the hot gas---and only a small fraction of this needs to be diverted to create the halo.

Peter Tribble,