Are Cluster Radio Relics Revived Fossil Radio Cocoons?
  A new model for the, so called, `cluster radio relics' is presented (Ensslin & Gopal-Krishna 2000). Fossil radio cocoons, resulting from the former activity of radio galaxies, should contain a low energy relativistic electron population and magnetic fields. Electrons with an age of even up to 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 a merging event of galaxy clusters, or by the accretion onto galaxy 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. 
Torsten A. Ensslin (MPI f. Astrophysik, Garching, Germany)   Gopal-Krishna (National Centre for Radio Astrophysics, Pune, India)