IMPLICATIONS OF FERMI OBSERVATIONS FOR HADRONIC MODELS OF RADIO HALOS IN CLUSTERS OF GALAXIES

We analyze the impact of the Fermi non-detection of gamma-ray emission from clusters of galaxies on hadronic models for the origin of cluster radio halos. In hadronic models, the inelastic proton-proton collisions responsible for the production of the electron-positron population fueling the observed synchrotron radio emission yield a gamma-ray flux, from the decay of neutral pions, whose spectrum and normalization depend on the observed radio emissivity and on the cluster magnetic field. We thus infer lower limits on the average cluster magnetic field in hadronic models from the Fermi gamma-ray limits. We also calculate the corresponding maximal energy density in cosmic rays and the minimal-guaranteed gamma-ray flux from hadronic radio-halo models. We find that the observationally most interesting cases correspond to clusters with large radio emissivities featuring soft spectra. Estimates of the central magnetic field values for those clusters are larger than, or close, to the largest magnetic field values inferred from Faraday rotation measures of clusters, placing tension on the hadronic origin of radio halos. In most cases, however, we find that the Fermi data do not per se rule out hadronic models for cluster radio halos as the expected gamma-ray flux can be pushed below the Fermi sensitivity for asymptotically large magnetic fields. We also find that cosmic rays do not contribute significantly to the cluster energy budget for nearby radio-halo clusters.