will help to understand the nature of the gamma-ray emission. Our study indicates that gamma rays from dark matter annihilation with a high particle mass can be distinguished from a cosmic ray spectrum even for fairly faint sources. Discriminating a cosmic ray spectrum from a light dark matter particle will be instead much more difficult, and will require long observations and/or a bright source. While the gamma-ray emission from our simulated clusters is extended, determining the spatial distribution with Fermi will be a challenging task requiring an optimal control of the backgrounds.'
author:
- 'Tesla E. Jeltema$^1$[^1], John Kehayias$^2$ and Stefano Profumo$^{2,3}$'
title: 'Gamma Rays from Clusters and Groups of Galaxies: Cosmic Rays versus Dark Matter'
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Introduction
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Clusters and groups of galaxies are the largest gravitationally bound matter structures observed in the Universe. Although these objects are expected to host several high energy phenomena, the resulting electromagnetic non-thermal emission is far from being fully understood [@brunetti; @heclusters]. A hallmark of the occurrence of non-thermal phenomena in these large structures is the detection, in numerous clusters, of extended radio emission associated to the synchrotron losses of relativistic cosmic-ray electrons [@brunetti; @heclusters; @Berrington:2002bh; @Colafrancesco:1998us; @Reimer:2004au]. The acceleration of cosmic rays 