Radio relics

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.

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 inside the plasma as well as the momenta of the relativistic particles.

In contrast to the radio gischt, the phoenices have a steep curved spectrum indicating an old population of electrons.

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 Brüggen using the Giant Metrewave Radio Telescope (GMRT) in India.