Dark Matter Substructure and Gamma‐Ray Annihilation in the Milky Way Halo

We present initial results from ``Via Lactea'', the highest resolution simulation to date of Galactic CDM substructure. It follows the formation of a Milky Way-size halo with Mvir=1. 8x10¹2 Msun in a WMAP 3-year cosmology, using 234 million particles. Over 10, 000 subhalos can be identified at z=0: Their cumulative mass function is well-fit by N (>Msub) = 0. 0064 (Msub/Mvir) ^ (-1) down to Msun=4x10⁶ Msun. The total mass fraction in subhalos is 5. 3%, while the fraction of surface mass density in substructure within a projected distance of 10 kpc from the halo center is 0. 3%. Because of the significant contribution from the smallest resolved subhalos, these fractions have not converged yet. Sub-substructure is apparent in all the larger satellites, and a few dark matter lumps are resolved even in the solar vicinity. The number of dark satellites with peak circular velocities above 10 km/s (5 km/s) is 124 (812): of these, 5 (26) are found within 0. 1 Rvir, a region that appeared practically smooth in previous simulations. The neutralino self-annihilation gamma-ray emission from dark matter clumps is approximately constant per subhalo mass decade. Therefore, while in our run the contribution of substructure to the gamma-ray luminosity of the Galactic halo amounts to only 40% of the total spherically-averaged smooth signal, we expect this fraction to grow significantly as resolution is increased further. An all-sky map of the expected annihilation gamma-ray flux reaching a fiducial observer at 8 kpc from the Galactic center shows that at the current resolution a small number of subhalos start to be bright enough to be visible against the background from the smooth density field surrounding the observer.