Gamma-ray probe of cosmic-ray pressure in galaxy clusters and cosmological implications

Cosmic rays produced in cluster accretion and merger shocks provide pressure to the intracluster medium (ICM) and affect the mass estimates of galaxy clusters. Although direct evidence for cosmic-ray ions in the ICM is still lacking, they produce γ-ray emission through the decay of neutral pions produced in their collisions with ICM nucleons. We investigate the capability of the Gamma-ray Large Area Space Telescope (GLAST) and imaging atmospheric Čerenkov telescopes (IACTs) for constraining the cosmic-ray pressure contribution to the ICM. We show that GLAST can be used to place stringent upper limits, a few per cent for individual nearby rich clusters, on the ratio of pressures of the cosmic rays and thermal gas. We further show that it is possible to place tight (. 10%) constraints for distant (z . 0.25) clusters in the case of hard spectrum, by stacking signals from samples of known clusters. The GLAST limits could be made more precise with the constraint on the cosmic-ray spectrum potentially provided by IACTs. Future γ-ray observations of clusters can constrain the evolution of cosmic-ray energy density, which would have important implications for cosmological tests with upcoming X-ray and Sunyaev-Zel’dovich effect cluster surveys.