Evidence for extended gamma-ray emission from galaxy clusters

摘要

We report evidence for extended gamma-ray emission from the Virgo, Fornax andComa clusters based on a maximum-likelihood analysis of the 3-year Fermi-LATdata. For all three clusters, excess emission is observed within three degreesof the center, peaking at the GeV scale. This emission cannot be accounted forby known Fermi sources or by the galactic and extragalactic backgrounds. Ifinterpreted as annihilation emission from supersymmetric dark matter (DM)particles, the data prefer models with a particle mass in the range 20-60 GeVannihilating into the b-bbar channel, or 2-10 GeV and >1 TeV annihilating intomu-mu final states. Our results are consistent with those obtained by Hooperand Linden from a recent analysis of Fermi-LAT data in the region of theGalactic Centre. An extended DM annihilation profile dominated by emission fromsubstructures is preferred over a simple point source model. The significanceof DM detection is 4.4 sigma in Virgo and lower in the other two clusters. Wealso consider the possibility that the excess emission arises from cosmic ray(CR) induced gamma-rays, and infer a CR level within a factor of three of thatexpected from analytical models. However, the significance of a CR component islower than the significance of a DM component, and there is no need for such aCR component in the presence of a DM component in the preferred DM mass range.We also set flux and cross-section upper limits for DM annihilation into theb-bbar and mu-mu channels in all three clusters.