Anisotropy probe of galactic and extra-galactic Dark Matter annihilations

TL;DR

This paper analyzes gamma-ray signals from Dark Matter annihilations in the Milky Way and beyond, highlighting the dominance of local substructures in the flux and potential detectability of anisotropies by Fermi LAT.

Contribution

It provides a self-consistent model of galactic and extragalactic DM substructures and predicts gamma-ray anisotropies detectable by current instruments.

Findings

MW substructures dominate gamma-ray flux at angles >1 degree

Potential detection of DM anisotropies by Fermi LAT at low multipoles

Extra-galactic annihilation contribution is negligible across scales

Abstract

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 show that the annihilation flux from substructures in the MW dominates over all the other components for angles larger than O(1) degrees from the Galactic Center, unless an extreme prescription is adopted for the substructures concentration. We also compute the angular power spectrum of gamma-ray anisotropies and find that, for an optimistic choice of the particle physics parameters, an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, l<100, where the dominant contribution comes from MW substructures with mass M>10^4 solar masses. For the substructures models we have adopted, we find that the contribution of extra-galactic annihilations is instead negligible at all scales.