4 v 1 3 0 Se p 19 97 DIFFUSE RADIO EMISSION IN THE COMA CLUSTER

The Coma cluster is peculiar in the radio domain owing to the presence of cluster-wide diffuse sources: the radio halo Coma C, the relic source 1253+275, and the bridge between them. Diffuse radio sources in clusters are a rare and poorly understood phenomenon. We summarize here the properties of the diffuse radio sources in Coma and their relation to other cluster properties. 1 Radio emission in Coma The radio emission in clusters of galaxies is due in the majority of cases to the individual radio emitting galaxies. The Coma cluster is indeed characterized by the presence of 29 cluster radio galaxies, of different optical morphological types. Moreover, the Coma cluster is permeated by diffuse radio emission, which is not identified with any individual galaxy and makes the Coma cluster exceptional in the radio domain. Cluster-wide diffuse sources witness the existence of relativistic electrons and large scale magnetic field, associated with the intergalactic medium. They are known to be present so far only in a few clusters, despite many efforts to detect them. The diffuse radio emission in Coma includes the central radio source Coma C, the peripheral diffuse source 1253+275, and the bridge of very low brightness between them. A radio image of the Coma cluster is presented in Fig. 1. A Hubble constant H 0 = 50 km s −1 Mpc −1 is used throughout. The radio galaxies: There are 29 cluster galaxies 1 showing radio emission down to a power level of ∼10 21 W Hz −1 : 10 are Ellipticals, 3 S0 + E/S0, and 16 Spirals + Irregulars. Among the extended radio galaxies identified with ellipticals, the dominant radio structure is the " tailed " structure, typically found in clusters, and determined by the drag exerted by the intracluster gas on the radio emitting plasma ejected from moving galaxies. In particular, the giant central galaxy NGC 4874 is known to be associated with a small (∼15 kpc) tailed radio source 2 , which indicates that the galaxy is not stationary at the cluster center. Also the giant peripheral galaxy NGC 4839 shows a tailed radio morphology 1 , with the low brightness tails oriented away from the cluster center. A prominent tailed radio source, extended about 300 kpc, is associated with NGC 4869 3 , located at ∼4 ′ from NGC 4874. Finally, the other tailed radio galaxies in Coma are NGC 4849