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 '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 ). Rogstad et al.  however pointed out that radio-galaxies in clusters are often associated with cDs. Ko  estimated an average of only one bright radio-galaxy per cluster.
In their search for clusters of galaxies around radio-sources, Bahcall et al.  and Bahcall & Bahcall  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  extended the quasar-cluster association up to redshifts z ~ 0.5. In 1980 Stockton et al.  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.  and Gursky et al. , Miley et al.  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.  detected the extended radio-source Coma C at 408 MHz, in the direction of the Coma cluster. Willson  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 '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  and Roland et al. .
Cluster radio-halos were as difficult to model, as they were to find. A first attempt was done by Jaffe , 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  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.  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  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).