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-sized halo with M-halo = 1.8; 10(12) M-circle dot in a WMAP three-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(> M-sub) 0.0064 (M-sub/M-halo)(-1) down to M-sub = 4 x 10(6) M-circle dot. 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(-1) (5 km s(-1)) is 124 (812): of these, five (26) are found within 0.1r(vir), 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.