7.3. The Radio Halo Coma C
A complete history of the Coma structure would not be complete without mentioning the radio-halo of Coma. In fact, theory and observations suggest that it is related to the subclustering in Coma.
Cluster radio-halos are very rare; Hanisch  searched for Coma-like radio-halos in 72 nearby Abell clusters and did not find any! So, Coma is rather exceptional in this respect (there are only ~ 10 cluster radio-halos known, see Feretti, these proceedings). Its radio-halo, Coma C, was first detected at 408 MHz by Large et al. , as an extended source of 45' size at the Coma centre. Willson showed that Coma C could not be produced by the integrated radiation from normal galaxies. Kim et al.  and Venturi et al.  found that Coma C extends to the SW, and Giovannini et al.  proposed the existence of a unique source extending from Coma C to 1253+274, passing through the SW group around NGC 4839 (see Section 7.1). Recently, Feretti et al.  measured a magnetic field associated with the cluster of ~ 8.5 µG, tangled on scales 1 kpc.
The theorists had trouble in explaining the energy source of Coma C (see, e.g., Tribble ). The radio-halo can be powered by relativistic electrons moving in a magnetic field. Cluster radio-galaxies can provide the relativistic electrons, but the strength of the magnetic field and the large extent of the radio-halo imply that the electrons must be re-accelerated far from their sources.
In the currently best model, recent (~ 108 years) subcluster collisions provide the re-acceleration energy (see Tribble ). However, many clusters contain substructures, and only a few clusters contain radio-halos, so the situation is not so simple (see Feretti's contribution in these proceedings for a discussion on this topic).