Limits to extragalactic cosmic rays from gamma-ray fluxes

Models involving the production of cosmic-ray protons and heavier nuclei of energies in the range 1-10 GeV in extragalactic space are considered and the fluxes of γ rays expected from their interactions with gas in clusters of galaxies are derived. Comparison with observation allows limits to be set on the fraction of the cosmic-ray flux detected at the Earth which is derived from extragalactic sources. It is found that for all reasonable models of extragalactic origin an excess of γ-ray flux would result; thus, the bulk of the 1-10 GeV cosmic rays detected appear to be of Galactic origin. (author)