The Clumpiness of Cold Dark Matter: Implications for the Annihilation Signal

We examine the expected signal from annihilation events in realistic cold dark matter halos. If the WIMP is a neutralino, with an annihilation cross-section predicted in minimal SUSY models for the lightest stable relic particle, the central cusps and dense substructure seen in simulated halos may produce a substantial flux of energetic gamma rays. We derive expressions for the relative flux from such events in simple halos with various density profiles, and use these to calculate the relative flux produced within a large volume as a function of redshift. This flux peaks when the first halos collapse, but then declines as small halos merge into larger systems of lower density. Simulations show that halos contain a substantial amount of dense substructure, left over from the incomplete disruption of smaller halos as they merge together. We calculate the contribution to the flux due to this substructure, and show that it can increase the annihilation signal substantially. Overall, the present-day flux from annihilation events may be an order of magnitude larger than predicted by previous calculations. We discuss the implications of these results for current and future gamma-ray experiments.