Radio Relics are diffuse

synchrotron A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed p ...

radio sources found in the peripheral regions of galaxy clusters. As in the case of radio halos, they do not have any obvious

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

counterpart, but their shapes are much more elongated and irregular compared to those of

radio halos Radio halos are large-scale sources of diffuse radio emission found in the center of some, but not all, galaxy clusters.Feretti, L., and G. Swarup. "The Universe at Low Radio Frequencies." Proceedings of IAU Symposium. Vol. 199. 2002. There are ...

. Their energy distribution is steep (much more energy at low radio frequency than at high radio frequency), with hints of a distribution of different ages for the emitting electrons across the whole dimension of the emitting region. Radio relics can be divided into two main groups: Cluster radio shocks or radio gischt are large elongated, often

Mpc MPC, Mpc or mpc may refer to: Astronomy * Megaparsec (Mpc), unit of length used in astronomy * Minor Planet Center, Smithsonian Astrophysical Observatory ** ''Minor Planet Circulars'' (MPC, M.P.C. or MPCs), astronomical publication from the Minor ...

-sized, radio sources located in the periphery of merging clusters. They probably trace shock fronts in which particles are accelerated via the diffusive shock acceleration mechanism. Among them are double-relics with the two relics located on both sides of a cluster center. Their integrated radio spectrum usually follows a single power-law. Radio phoenices are related to radio-loud active galactic nuclei (AGN). Fossil radio plasma from a previous episode of AGN activity is thought to be compressed by a merger shock wave which boosts, both, the

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

inside the

plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...

as well as the momenta of the relativistic particles. As a result, the radio plasma brightens in synchrotron emission. In contrast to the radio gischt, the phoenices have a steep curved spectrum indicating an old population of electrons. The sizes of relics and the distance to the cluster centre vary significantly. Examples for radio relics with sizes of 1 Mpc or even larger have been observed in Coma (the prototype relic source 1253 + 275), Abell 2255 and

Abell 2256 Abell 2256 is a rich nearby galaxy cluster in the Abell catalogue that exhibits a population of ~ 100 – 200 kpc long steep spectrum synchrotron filaments surrounding the cluster center with significant evidence of merger activity deduced by th ...

, which contain both a relic and a halo (as do Abell 225, Abell 521, Abell 754, Abell 1300, Abell 2255 and Abell 2744). The cluster Abell 3667 contains two very luminous, almost symmetric relics with a separation of more than 5 Mpc, as does ZwCl 2341.1+0000, Abell 2345, Abell 1240, and ZwCl 0008.8+5215. The relic with the best evidence for shock acceleration found to date is located in the northern outskirts of the merging galaxy cluster CIZA J2242.8+5301. This relic has been nicknamed the sausage and has been discovered by Reinout van Weeren and Marcus Bruggen using the Giant Metrewave Radio Telescope (GMRT) in India.

References

* * * * * * {{DEFAULTSORT:Radio Relics * Large-scale structure of the cosmos