3.7. Radio components

The idea that clusters could be associated with extragalactic radio-sources dates back to 1960. At that time, it was generally thought that galaxy-galaxy interactions and merging were a pre-requisite for radio-source activity in galaxies. Spitzer & Baade [430]'s work had shown that collisions must be frequent among cluster galaxies. It was then quite natural to suggest that extragalactic radio-sources could be associated with galaxy clusters (Minkowski [307]). Rogstad et al. [377] however pointed out that radio-galaxies in clusters are often associated with cDs. Ko [261] estimated an average of only one bright radio-galaxy per cluster.

In their search for clusters of galaxies around radio-sources, Bahcall et al. [38] and Bahcall & Bahcall [37] found evidence for significant galaxy clustering around quasars at z ~ 0.1-0.2. In those years (the early 70's) the importance of this discovery was that it provided evidence for a common origin of the galaxy and the quasar redshifts. If the galaxy redshifts were cosmological, so were the redshifts of quasars. Rózyczka [394] extended the quasar-cluster association up to redshifts z ~ 0.5. In 1980 Stockton et al. [435] showed that while giant radio-galaxies are often found in clusters, quasars live in intermediate density environments, like galaxy groups.

A class of radio-sources that are exclusively found in clusters are the head-tail radio-sources. Immediately after the IC gas discovery by Meekins et al. [299] and Gursky et al. [201], Miley et al. [304] were able to model this peculiar radio morphology in terms of radio-trails of galaxies moving through the dense IC gas.

In 1959 Large et al. [267] detected the extended radio-source Coma C at 408 MHz, in the direction of the Coma cluster. Willson [498] showed Coma C to be a wide 40 arcmin diffuse emission, not originating from the integrated emission of individual galaxies. If located at the distance of the Coma cluster, the size of Coma C corresponds to 1.2 Mpc. For this reason, Willson named it ``the halo''.

In those days, Coma was still considered as the typical cluster. However, it was soon clear that clusters with radio-halos are rare. Hanisch et al. [205, 203] could list only four clusters with detected radio-halos, and Jaffe & Rudnick [241]'s extensive search for radio-halos in 32 clusters did not detect any. Eventually, two other cluster radio-halos were discovered in those years, by Harris & Miley [206] and Roland et al. [379].

Cluster radio-halos were as difficult to model, as they were to find. A first attempt was done by Jaffe [240], who suggested that the radio-halo could be created from the leakage of electrons out of radio-galaxies, but the model could not really account for the wide distribution of the radio-emission. Roland [378] proposed an in situ acceleration of relativistic electrons by magnetic field fluctuations generated in the wakes of moving galaxies. A hint to the nature of radio-halos came from their rarity. In 1979 Smith et al. [423] remarked that both Coma and Abell 2319 (two radio-halo clusters) have too high an X-ray temperature for their velocity dispersion. Three years later, Hanisch [204] and Vestrand [483 noted that the rare clusters harbouring a radio-halo have many other similar properties. These are: anomalous high X-ray temperatures for their galaxy velocity dispersions, low spiral contents, intermediate Bautz-Morgan types, large X-ray core-radii, smooth X-ray distributions, without the central peak typical of cD clusters. Hanisch and Vestrand suggested that the presence of a radio-halo could be related to a short-lived dynamical configuration, thus anticipating modern scenarios (see, e.g., FERETTI, these proceedings).