Gamma Rays from Intergalactic Shocks

Abstract

Structure formation in the intergalactic medium (IGM) produces large-scale, collisionless shock waves, in which electrons can be accelerated to highly relativistic energies. Such electrons can Compton-scatter cosmic microwave background photons up to gamma-ray energies. We study the radiation emitted in this process using a hydrodynamic cosmological simulation of a LambdaCDM 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. Assuming that a fraction xi(e)=0.05 of the shock thermal energy is transferred to the population of accelerated relativistic electrons, as inferred from collisionless nonrelativistic shocks in the interstellar medium, we find that the energy flux of this radiation, epsilon(2)(dJ/depsilon)similar or equal to50-160 eV cm(-2) s(-1) sr(-1), constitutes similar to10% of the extragalactic gamma-ray background flux. The associated gamma-ray point sources are too faint to account for the similar to60 unidentified EGRET gamma-ray sources, but GLAST should detect and resolve several gamma-ray sources associated with large-scale IGM structures for xi(e)similar or equal to0.03 and many more sources for larger xi(e). 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 postshock magnetic fields. We predict that GLAST and Cerenkov telescopes such as MAGIC, VERITAS, and HESS should resolve gamma-rays from nearby (redshifts zless than or similar to0.01) rich galaxy clusters, perhaps in the form of a similar to5-10 Mpc diameter ringlike emission tracing the cluster accretion shock, with luminous peaks where the ring intersects galaxy laments detectable even at zsimilar or equal to0.025.