  Ultra-high energy cosmic rays (UHECRs) accelerated in the jets of active
galactic nuclei can accumulate in high magnetic field, ~100 kpc-scale regions
surrounding powerful radio galaxies. Photohadronic processes involving UHECRs
and photons of the extragalactic background light make ultra-relativistic
electrons and positrons that initiate electromagnetic cascades, leading to the
production of a gamma-ray synchrotron halo. We calculate the halo emission in
the case of Cygnus A and show that it should be detectable with the Fermi Gamma
ray Space Telescope and possibly detectable with ground-based gamma-ray
telescopes if radio galaxies are the sources of UHECRs.
