Cosmic rays from active galactic nuclei and in metagalactic space

Studies of extensive air showers have shown that cosmic rays at energies above 1019 eV are likely to be extragalactic in origin. The energy density above 1019 eV is 3 x 10-8 eV cm-3, and the exponent of the integral energy spectrum is -1.4 +- 0.1. For estimating the mean energy density of cosmic-ray protons in the supercluster, above 1 GeV, we also assume a power output for protons similar to that for relativistic electrons in active galactic nuclei. The mean energy density of relativistic electrons in the supercluster can be estimated from the nonthermal output of electromagnetic radiation. For cosmic rays in the supercluster, we obtain an energy density of 10/sup -4plus-or-minus1/ eV cm-3. Our estimate for this value is supported by the following internal consistency argument: the proton intensity at 109 eV (subject to the foregoing condition on power output), when extended with the exponent -1.4, smoothly joins the extragalactic spectrum at 1019 eV, having the same slope. It is noteworthy that the exponent -1.4 is also that of galactic cosmic rays when corrected for rigidity-dependent leakage from the Galaxy