Gamma rays from galaxy clusters and their dominant members

Clusters of galaxies are a long-expected source class of diffuse γ-ray emission, but a detection of such is yet to be made. Point-like signals from their dominant mem- bers are observed, and provide fresh insight into the processes involved in feedback from active galaxies, which suppress cooling in cluster cores, and inject non-thermal particles into the intracluster medium. The case for multi-scale γ-ray emission via leptonic and hadronic channels subse- quent to cosmic-ray acceleration, dark matter annihilation, and processes driven by the active nuclei of brightest cluster galaxies (BCGs), motivates the work. A temporal analysis of the source associated with the BCG NGC1275 is carried out; the high-energy (HE) emission (observed with Fermi) is compared with 1.3mm SMA data. No correlation is found on short timescales, however a gradual rising trend is common in both light curves, in agreement with the historical activity of the source, and suggestive of long-term variation in the fuelling of the black hole. Very-high-energy emission recorded with H.E.S.S. over the past decade from the BCG M87 is analysed spectrally, and compared with the HE activity of the source, and with NGC1275. Three samples of candidate HE sources are constructed: 114 galaxy clusters con- taining a radio-bright BCG (motivated by the detection of NGC1275 and M87), 105 hosting diffuse radio synchrotron emission, and the 90 most promising targets for dark matter annihilation signatures. Samplewise, candidate fields output from standard Fermi analyses are stacked. The upper limit on the emission within the stacked target region is at least an order-of-magnitude more constraining than lim- its on individual clusters in each case. A subset of local clusters are investigated as potential extended sources, yielding hints of diffuse emission from Perseus, and several individual fields are found to contain significant emission.