Large-scale components of radio galaxies in gamma rays

Abstract

The populations of relativistic electrons and high-energy cosmic rays in the extended components of radio-loud active galaxies–kiloparsec-scale jets, hotspots and lobes–make them, in principle, interesting sources of gamma-ray photons up to the highest (TeV) energies. Here I discuss the few detections of extended gamma-ray emission with Fermi and show what constraints they put on the physical conditions in radio galaxies. Although detections of radio galaxies at TeV energy are rare, I show that the information we do have can already put interesting constraints on radio galaxy physics: the CTA should have the sensitivity and resolution to improve this situation considerably, and to resolve the expected arcmin-scale gamma-ray jet in the nearest radio galaxy, Centaurus A. Finally, I review the connection between the gamma-ray observations and the acceleration of ultra-high-energy cosmic rays, and suggest that, while radio galaxies may well be the sources of the highest-energy cosmic rays observed, they may constitute a ‘disappointing’ population in the sense that only very nearby sources, such as Cen A, will ever be identified as discrete cosmic-ray sources.