Limits to extragalactic cosmic rays from gamma-ray fluxes

Abstract

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 gamma 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 gamma -ray flux would result; thus, the bulk of the 1-10 GeV cosmic rays detected appear to be of Galactic origin.