The first hint towards their origin came from X-ray images of galaxy clusters containing these 'cluster radio relics'.
Clusters of galaxies contain dilute and hot, X-ray emitting gas in the space between the galaxies.
In most clusters this gas is symmetrically distributed about their centres. These deformations indicate that violent collisions of two clusters of galaxies are taking place. During such collisions giant shock waves are formed, in which the cluster gas is compressed and heated. Enough energy is released in these shock waves to power the emission of cluster radio relics. In several cases now, detailed investigations reveal that relics are indeed located where the presence of a shock wave can be deduced from X-ray observations.