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 [64] 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. [83], 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. [80] and Venturi et al. [146] found that Coma C extends to the SW, and Giovannini et al. [56] 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. [43] 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 [139]). 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 [140]). 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).