New Limits on Gamma-Ray Emission From Galaxy Clusters

Galaxy clusters are predicted to produce γ-rays through cosmic ray interactions and/or dark matter annihilation, potentially detectable by the Fermi Large Area Telescope ( Fermi -LAT). We present a new, independent stacking analysis of Fermi -LAT photon count maps using the 78 richest nearby clusters ( z < 0.12) from the Two Micron All Sky Survey cluster catalog. We obtain the lowest limit on the photon flux to date, 2.3 × 10 −11 photons cm −2 s −1 (95% confidence) per cluster in the 0.8–100 GeV band, which corresponds to a luminosity limit of 3.5 × 10 44 photons s −1 . We also constrain the emission limits in a range of narrower energy bands. Scaling to recent cosmic ray acceleration and γ-ray emission models, we find that cosmic rays represent a negligible contribution to the intra-cluster energy density and gas pressure.