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We present initial results from ``Via Lactea'', the highest resolutionsimulation to date of Galactic CDM substructure. It follows the formation of aMilky Way-size halo with Mvir=1.8x10^12 Msun in a WMAP 3-year cosmology, using234 million particles. Over 10,000 subhalos can be identified at z=0: Theircumulative mass function is well-fit by N(>Msub)= 0.0064 (Msub/Mvir)^(-1) downto Msun=4x10^6 Msun. The total mass fraction in subhalos is 5.3%, while thefraction of surface mass density in substructure within a projected distance of10 kpc from the halo center is 0.3%. Because of the significant contributionfrom the smallest resolved subhalos, these fractions have not converged yet.Sub-substructure is apparent in all the larger satellites, and a few darkmatter lumps are resolved even in the solar vicinity. The number of darksatellites 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 practicallysmooth in previous simulations. The neutralino self-annihilation gamma-rayemission from dark matter clumps is approximately constant per subhalo massdecade. Therefore, while in our run the contribution of substructure to thegamma-ray luminosity of the Galactic halo amounts to only 40% of the totalspherically-averaged smooth signal, we expect this fraction to growsignificantly as resolution is increased further. An all-sky map of theexpected annihilation gamma-ray flux reaching a fiducial observer at 8 kpc fromthe Galactic center shows that at the current resolution a small number ofsubhalos start to be bright enough to be visible against the background fromthe smooth density field surrounding the observer.

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