Radio galaxies and the origin of ultra-high energy cosmic rays

Among the possible sources of ultra-high energy cosmic rays are the hot-spots of Fanaroff-Rlley II radio galaxies. These regions meet the requirements of size, magnetic field intensity and presence of strong shocks for accelerating particles up to energies that exceed 1021 eV. On the other hand, the interaction with the photons of the Cosmic Microwave Background radiation, the Greisen-Zatsepin-Kuz'min effect, dictates that the sources of particles with energy above 4×1019 eV must be within 130 Mpc, at most. There are not very many FR II radio galaxies within this distance, i.e. 15 objects. Once the statistics of events for detection of cosmic rays at these energies will have reached reasonable levels (see the Pierre Auger Experiment [J.W. Cronin, Nucl. Phys. B 138 (2005) 465]), looking for the arrival directions will either confirm or rule out the role of FR II hot-spots as sources of ultra-high energy cosmic rays.