Radio halo
Encyclopedia
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.
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
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
Radio relics
Radio Relics are diffuse synchrotron radio emission found in the peripheral regions of galaxy clusters. Similar to the case of radio halos, they do not have any obvious galaxy counterpart, but their shapes are much more elongated and irregular compared to those of radio halos...
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.