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. Their energy distribution is steep (much more energy at low radio frequency that at high radio frequency), with hints of a distribution of different ages for the emitting electrons across the whole dimension of the emitting region. Their cause is still debated, but they may come to exist due to the direct acceleration of relativistic electrons in case of a large scale shock wave, and/or to the adiabatic compression of a pre-existing mildly relativistic plasma, which is compressed and re-energised by the passage of a compressive wave.

References

* Ensslin T., Biermann P., Klein U., Kohle S., 1998, A&A 332, 395
* Goldsmith O., Rephaeli Y., 1994, ApJ 431, 586
* Orru E., Feretti L., Govoni F., Murgia M., Giovannini G., Brunetti G., Setti G., 2006, AN 327,565