A Statistical Detection of Gamma‐Ray Emission from Galaxy Clusters: Implications for the Gamma‐Ray Background and Structure Formation

Abstract

The origin of the diffuse extragalactic high-energy gamma-ray background (EGRB) filling the universe remains unknown. The spectrum of this extragalactic radiation, as measured by EGRET on board the Compton Gamma Ray Observatory, is well fitted by a power law across nearly four decades in energy, from 30 MeV to 100 GeV. It has been estimated that not more than a quarter of the diffuse gamma-ray background could be due to unresolved point sources. Recent studies have suggested that much of the diffuse background could originate from the upscatter of cosmic microwave background photons by relativistic electrons produced by shock waves in the intergalactic medium during large-scale structure formation. In this work, we search for evidence of gamma-ray emission associated with galaxy clusters by cross-correlating high Galactic latitude EGRET data with Abell clusters. Our results indicate a possible association of emission with clusters at a � 3 � level. For a subset of the 447 richest (R � 2) clusters, the mean surface brightness excess is 1:2 � 10 � 6 photons cm � 2 s � 1 sr � 1 (>100 MeV), corresponding to a typical nonthermal bolometric luminosity of L� � 1 � 10 44 ergs s � 1 . Extrapolating this measurement and assuming no evolution, we conservatively estimate that � 1%–10% of the EGRB could originate from clusters with z < 1. For this cluster population, the predicted nonthermal luminosity is in excellent agreement with our measurement, suggesting that the clusters have experienced mass accretion within the last 10 9 yr. If correct, then future gamma-ray missions, such as the Gamma-Ray Large Area Space Telescope, should be able to directly detect nearby galaxy clusters. Subject headings: galaxies: clusters: general — gamma rays: observations — large-scale structure of universe