The presence of magnetic fields in clusters is directly demonstrated by the existence of large-scale diffuse synchrotron sources, that have no apparent connection to any individual cluster galaxy and are therefore associated with the ICM. These radio sources have been classified as radio halos, relics and mini-halos depending on their morphology and location. Radio halos, relics and mini-halos are not a common phenomenon in clusters and until recently they were known to exist in only a handful of clusters of galaxies . This was a consequence of the fact that they show low surface brightness, large size and steep spectrum, thus they are difficult to reveal. Extended, low-brightness structures are most easily detected with filled-aperture telescopes, but the low resolving power of single-dish radio telescopes increases problems with confusion and can create an apparent wide source from a blend of weak discrete radio sources. Observations made with interferometers have the angular resolution necessary to separate the individual radio galaxies, but generally lack information from short spacing, thus hindering the detection of extended low-brightness structures.
The number of clusters with known diffuse sources has increased in the last few years to around 50, thanks to the improved sensitivity of radio telescopes and the existence of deep surveys. New halo and relic candidates were found from searches in the NRAO VLA Sky Survey (NVSS ) by Giovannini et al. , in the Westerbork Northern Sky Survey (WENSS ) by Kempner and Sarazin , in the Sidney University Molonglo Sky Survey (SUMSS ) by Hunstead et al.  and in the survey of the Sharpley Concentration by Venturi et al. .
The presence of these large regions of diffuse synchrotron emission reveals a large-scale distribution of GeV relativistic electrons radiating in ~ µG magnetic fields, in the ICM.