Positional Coincidence between the High‐Latitude Steady Unidentified Gamma‐Ray Sources and Possibly Merging Clusters of Galaxies

We report an evidence for the first time that merging clusters of galaxiesare a promising candidate for the origin of high galactic-latitude, steadyunidentified EGRET gamma-ray sources. Instead of using past optical catalogs ofeye-selected clusters, we made a matched-filter survey of galaxy clusters over$4\arcdeg \times 4\arcdeg$ areas around seven steady unidentified EGRET sourcesat $|b|>45\arcdeg$ together with a 100 $\sq \arcdeg$ area near the SouthGalactic Pole as a control field. In total, 154 Abell-like cluster candidatesand 18 close pairs/groups of these clusters, expected to be possibly mergingclusters, were identified within estimated redshift $z_{est}\leq 0.15$. Fiveamong the seven EGRET sources have one or two cluster pairs/groups (CPGs)within 1$\arcdeg$ from them. We assess the statistical significance of thisresult by several methods, and the confidence level of the real excess ismaximally 99.8% and 97.8% in a conservative method. In contrast, we found nosignificant correlation with single clusters. In addition to the spatialcorrelation, we also found that the richness of CPGs associated with EGRETsources is considerably larger than those of CPGs in the control field. Theseresults imply that a part of the steady unidentified EGRET sources athigh-latitude are physically associated with close CPGs, not with singleclusters. We also discuss possible interpretations of these results. We arguethat, if these associations are real, they are difficult to explain by hadronicprocesses, but best explained by the inverse-Compton scattering by high energyelectrons accelerated in shocks of cluster formation, as recently proposed.Comment: 9 pages, 2 PostScript figures, uses emulateapj5.sty, added new analysis and discussion, ApJ accepte