Radio halo

This article is about the astronomical phenomenon. For the geological phenomenon, see radiohalo.

Halo of the Abell 1758 cluster shown in pink from the GMRT, overlaid with Chandra data shown in blue.

Radio halos are large-scale sources of diffuse radio emission found in the center of some, but not all, galaxy clusters. There are two classes of radio halos: mini-halos and giant radio halos. The linear size of giant radio halos is about 700kpc-1Mpc, whereas mini-halos are typically less than 500kpc. Giant radio halos are more often observed in highly X-ray luminous cluster samples than less luminous X-ray clusters ($\mathrm {L} _{X}\leq 10^{45}\mathrm {erg\ s} ^{-1}$) in complete samples. They have a very low surface brightness and do not have obvious galaxy counterparts (e.g. like radio galaxies have AGN counterparts), however, their morphologies typically follow the distribution of gas in the intra-cluster medium. Mini-halos however, while similar to giant halos, are found at the center of cooling core clusters but around a radio galaxy.

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.q 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.