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Radio halos are large-scale areas of radio emission found in clusters of galaxies. They do not have an obvious galaxy counterpart, as opposed, for example, to radio galaxies, which have AGN counterparts. Their cause is still debated, but they may come to exist due to reacceleration of mildly relativistic particles (electrons) happening during a merger event between galaxy clusters: the generated turbulent motions of the intra-cluster plasma drive Magneto-Hydrodynamical Waves, which couples with mildly relativistic particles (i.e. of energy on the level of 102MeV) and accelerate them up to energy of 10GeV or more. They are found at the centre of clusters. Radio relics are similar features found at the edge of clusters, even though their driving mechanism should be different (emitting electrons are more likely accelerated not by turbulence, but by shock waves instead).
They are likely to result from synchrotron radiation originating from electrons, moving in the intracluster magnetic field of around 0.1 - 3 μ G.
- Ferrari, Govoni, Schindler, Bykov, Rephaeli - Observations of extended radio emission in clusters - astro-ph 2007
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