Clumps and streams in the local dark matter distribution

The standard cosmological model includes the presumption that cold dark matter plays a major part in large-scale mass distribution in the Universe from the Big Bang to the present. Though successful in explaining large-scale events, until now simulations of galaxy formation using the cold dark matter model have failed to resolve certain smaller-scale structures. Now Diemand et al. have simulated the assembly of the dark matter 'halo' of the Milky Way at much better resolution than has been possible previously. The model then produces thousands of clumps surviving within the inner halo, some of them in the vicinity of the Solar System. In cold dark matter cosmological models, structures form and grow by merging of smaller units, previous simulations have shown that such merging is incomplete as the inner cores of halos survive and orbit as 'subhalos' within their hosts. This paper reports a simulation that resolves such substructure in the very inner regions of the Galactic halo. Hundreds of very concentrated dark matter clumps survive near the solar circle, as well as numerous cold streams. In cold dark matter cosmological models1,2, structures form and grow through the merging of smaller units3. Numerical simulations have shown that such merging is incomplete; the inner cores of haloes survive and orbit as ‘subhaloes’ within their hosts4,5. Here we report a simulation that resolves such substructure even in the very inner regions of the Galactic halo. We find hundreds of very concentrated dark matter clumps surviving near the solar circle, as well as numerous cold streams. The simulation also reveals the fractal nature of dark matter clustering: isolated haloes and subhaloes contain the same relative amount of substructure and both have cusped inner density profiles. The inner mass and phase-space densities of subhaloes match those of recently discovered faint, dark-matter-dominated dwarf satellite galaxies6,7,8, and the overall amount of substructure can explain the anomalous flux ratios seen in strong gravitational lenses9,10. Subhaloes boost γ-ray production from dark matter annihilation by factors of 4 to 15 relative to smooth galactic models. Local cosmic ray production is also enhanced, typically by a factor of 1.4 but by a factor of more than 10 in one per cent of locations lying sufficiently close to a large subhalo. (These estimates assume that the gravitational effects of baryons on dark matter substructure are small.)