Diffuse extended radio sources are difficult to detect because of their low surface brightness and large size. In observations performed with interferometers, the lack of short spacings prevents the detection of large scale structures. Observations with filled aperture telescopes, on the other hand, do not have high enough resolution to distinguish a real diffuse emission from a blend of point sources. Despite of these observational difficulties, several surveys were undertaken to detect radio halos and determine how common they are (see for example: Jaffe & Rudnick 1979; Cane et al. 1981; Hanisch 1982b), but the general conclusion was that such sources are extremely rare. In the review paper by Feretti & Giovannini (1996), the reported number of known halo and relic sources was still very low; only a dozen of them were at that time unambiguously detected. The number of known diffuse sources has increased recently to about 40 objects, thanks to the improved sensitivity of radio telescopes and the existence of deep surveys.
In the following we report the observational results, in approximate chronological order.
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