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

The origin of the diffuse extragalactic, high-energy gamma-ray background(EGRB) filling the Universe remains unknown. The spectrum of this extragalacticradiation, as measured by the EGRET on-board CGRO, is well-fit by a power lawacross nearly four decades in energy, from 30 MeV to 100 GeV. It has beenestimated that not more than a quarter of the diffuse gamma-ray backgroundcould be due to unresolved point sources. Recent studies have suggested thatmuch of the diffuse background could originate from the up-scatter of CMBphotons by relativistic electrons produced by shock waves in the IGM duringlarge-scale structure formation. In this work we search for evidence ofgamma-ray emission associated with galaxy clusters by cross-correlating highGalactic latitude EGRET data with Abell clusters. Our results indicate apossible association of emission with clusters at a >3-sigma level. For asubset of the 447 richest (R>=2) clusters the mean surface brightness excess is1.2x10^-6 ph cm^-2 s^-1 sr^-1 (>100MeV), corresponding to a typical non-thermalbolometric luminosity of L_{gamma} ~ 1x10^44 erg s^-1. Extrapolating thismeasurement and assuming no evolution we conservatively estimate that ~ 1-10 ofthe EGRB could originate from clusters with z<1. For this cluster populationthe predicted non-thermal luminosity is in excellent agreement with ourmeasurement, suggesting that the clusters have experienced mass accretionwithin the last 10^9 yrs. If correct, then future gamma-ray missions, such asGLAST should be able to directly detect nearby galaxy clusters.Comment: Accepted ApJ, 26 pages, 9 figures, figures 1,2, and 3 are bitmappe