Clumps and streams in the local dark matter distribution


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.)

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Figure 1: Via Lactea II projected dark matter squared-density map.
Figure 2: Density profiles of main halo and subhaloes.
Figure 3: Subhalo and sub-subhalo abundances.


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It is a pleasure to thank B. Messer and the Scientific Computing Group at the National Center for Computational Sciences for their help. The Via Lactea II simulation was performed at the Oak Ridge National Laboratory through an award from the US Department of Energy’s Office of Science as part of the 2007 Innovative and Novel Computational Impact on Theory and Experiment (INCITE) programme. Additional computations (initial conditions generation, code optimizations and smaller test runs) were carried out on the MareNostrum supercomputer at the BSC, on Columbia at NASA Ames and on the UCSC Astrophysics Supercomputer Pleiades. This work was supported by NASA and the Swiss National Science Foundation.

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Correspondence to J. Diemand.

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Diemand, J., Kuhlen, M., Madau, P. et al. Clumps and streams in the local dark matter distribution. Nature 454, 735–738 (2008).

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