Gamma-Rays from Intergalactic Shocks

Structure formation in the intergalactic medium (IGM) produces large-scale, collisionless shock waves, where electrons can be accelerated to highly relativistic energies. Such electrons can Compton scatter cosmic microwave background photons up to γ-ray energies. We study the radiation emitted in this process using a hydrodynamic cosmological simulation of a ΛCDM universe. The resulting radiation, extending beyond TeV energies, has roughly constant energy flux per decade in photon energy, in agreement with the predictions of Loeb & Waxman (2000). Assuming that a fraction ξe = 0.05 of the shock energy is transferred to the population of accelerated relativistic electrons, as inferred from collisionless non-relativistic shocks in the interstellar medium, we find that the energy flux of this radiation, ǫ(dJ/dǫ) ≃ 50− 160 eV cm s sr, constitutes ∼ 10% of the extragalactic γ-ray background flux. The associated γ-ray point-sources are too faint to account for the ∼ 60 unidentified EGRET γ-ray sources, but GLAST should detect and resolve several γ-ray sources associated with large-scale IGM structures for ξe ≃ 0.03, and many more sources for larger ξe. The intergalactic origin of the shock-induced radiation can be verified through a cross-correlation with, e.g., the galaxy distribution that traces the same structure. Its shock-origin may be tested by cross-correlating its properties with radio synchrotron radiation, emitted as the same accelerated electrons gyrate in post-shock magnetic fields. We predict that GLAST and the MAGIC telescope should resolve γ-rays from nearby (redshifts z . 0.01) rich galaxy clusters, in the form of a ∼ 5 − 10 Mpc diameter ring-like emission tracing the cluster accretion shock, with luminous peaks at its intersections with galaxy filaments detectable even at z ≃ 0.025. Subject headings: large-scale structure of universe — galaxies: clusters: general — gamma rays: theory — methods: numerical — radiation mechanisms: nonthermal — shock waves Department of Condensed Matter Physics, Weizmann Institute, Rehovot 76100, Israel; waxman,[email protected] Harvard-Smithsonian CFA, 60 Garden Street, Cambridge, MA 02138, USA Max-Plank-Institut für Astrophysik, Karl-Shwarzschild-Straße 1, 85740 Garching bei München, Germany