Are cluster radio relics revived fossil radio cocoons?

A new model for the so called cluster radio relics is presented (see Ensslin and Gopal-Krishna 2001 for more details). Fossil radio cocoons, resulting from the former activity of radio galaxies, should contain a low energy relativistic electron population and magnetic fields. Even electrons with ages as high as 2 Gyr can be re-accelerated adiabatically to radio- emitting energies, if the fossil radio plasma gets compressed in an environmental shock wave. Such a wave can be caused by merging events in galaxy clusters, or by accretion onto clusters. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low-frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation