Positional Coincidence between the High-latitude Steady Unidentified Gamma-ray Sources and Possibly Merging Clusters of Galaxies
We report evidence for the first time that merging clusters of galaxies are a promising candidate for the origin of high galactic-latitude, steady unidentified EGRET gamma-ray sources. Instead of using past optical catalogues of eye-selected clusters, we made a matched-filter survey of galaxy clusters over 4o × 4o areas around seven steady unidentified EGRET sources at |b|>45o together with a 100o area near the South Galactic Pole as a control field. In total, 154 Abell-like cluster candidates and 18 close pairs/groups of these clusters, expected to be possibly merging clusters, were identified within an estimated redshift zest<= 0.15. Five among the seven EGRET sources have one or two cluster pairs/groups (CPGs) within 1o from them. We assess the statistical significance of this result by several methods, and the confidence level of the real excess is maximally 99.8% and 97.8% in a conservative method. In contrast, we found no significant correlation with single clusters. In addition to the spatial correlation, we also found that the richness of CPGs associated with EGRET sources is considerably larger than those of CPGs in the control field. These results imply that a part of the steady unidentified EGRET sources at high-latitude are physically associated with close CPGs, not with single clusters.
8th Asian-Pacific Regional Meeting, Volume II
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