Dark Matter Annihilation in Substructures Revised

Upcoming γ-ray satellites will search for Dark Matter annihilations in Milky Way substructures (or ’clumps’). The prospects for detecting these objects strongly depend on the assumptions made on the distribution of Dark Matter in substructures, and on the distribution of substructures in the Milky Way halo. By adopting extreme prescriptions for these quantities, we bracket the number of sources that can be detected with upcoming experiments such as GLAST. We find that scenarios leading to a high number of detectable sources, as well as scenarios where micro-clumps (i.e. clumps with mass as small as 10M⊙) can be detected, are severely constrained by the diffuse γ-ray background detected by EGRET. At most a handful of large mass substructures, and no micro-clumps, can be detected (at 3σ, with a 1-year exposure time, by a GLAST-like experiment) in the most optimistic scenarios.