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

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.