Prospects of detecting gamma-ray emission from galaxy clusters: Cosmic rays and dark matter annihilations

We study the possibility for detecting gamma-ray emission from galaxy clusters. We consider (1) leptophilic models of dark matter (DM) annihilation that include a Sommerfeld enhancement (SFE), (2) different representative benchmark models of supersymmetric DM, and (3) cosmic-ray (CR) induced pion decay. Among all clusters/groups of a flux-limited x-ray sample, we predict Virgo, Fornax, and M49 to be the brightest DM sources and find a particularly low CR-induced background for Fornax. For a minimum substructure mass given by the DM free-streaming scale, cluster halos maximize the substructure boost for which we find a factor of 1000. Since regions around the virial radius dominate the annihilation flux of substructures, the resulting surface brightness profiles are almost flat. This makes it very challenging to detect this flux with imaging atmospheric Cherenkov telescopes since their sensitivity drops approximately linearly with radius and they typically have 5--10 linear resolution elements across a cluster. Assuming cold dark matter with a substructure mass distribution down to an Earth mass and using extended Fermi upper limits, we rule out the leptophilic models in their present form in 28 clusters, and limit the boost from SFE in M49 and Fornax to be 5. This corresponds to a limit on SFE in the Milky Way of 3, which is too small to account for the increasing positron fraction with energy as seen by PAMELA and challenges the DM interpretation. Alternatively, if SFE is realized in nature, this would imply a limiting substructure mass of M₋₈₌>10^4M_---a problem for structure formation in most particle physics models. Using individual cluster observations, it will be challenging for Fermi to constrain our selection of DM benchmark models without SFE. The Fermi upper limits are, however, closing in on our predictions for the CR flux using an analytic model based on cosmological hydrodynamical cluster simulations. We limit the CR-to-thermal pressure in nearby bright galaxy clusters of the Fermi sample to 10% and in Norma and Coma to 3%. Thus, we will soon start to constrain the underlying CR physics such as shock acceleration efficiencies or CR transport properties.