We study the flux and the angular power spectrum of gamma rays produced by dark matter (DM) annihilations in the Milky Way (MW) and in extra-galactic halos. The annihilation signal receives contributions from: (a) the smooth MW halo, (b) resolved and unresolved substructures in the MW, (c) external DM halos at all redshifts, including (d) their substructures. Adopting a self-consistent description of local and extra-galactic substructures, we compute the angular power spectrum of gamma-ray anisotropies and show that, quite independently from the prescription used for the concentration parameters $c(M,z)$, an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, $\ell \sim 100$, assuming a particle physics scenario that does not violate gamma-ray constraints from the Galactic center. The DM contribution to the angular power spectrum is dominated by MW substructures (with mass $M\ge {10}^4{M}_{\odot }$), while, at least for the substructures’ models that we have adopted, the contribution of extra-galactic annihilations is negligible at all scales. MW subhalos also dominate the annihilation flux in the majority of the sky, with the smooth galactic halo being brighter only in the very central region.

• Received 29 January 2009

DOI:https://doi.org/10.1103/PhysRevD.80.023518