Chapter

The Aquarius Project: Cold Dark Matter under a Numerical Microscope

DOI: 10.1007/978-3-540-69182-2_8

ABSTRACT The ‘Aquarius’ project currently performs the first ever one-billion particle simulation of a Milky Way-sized dark matter
halo, improving resolution by a factor of more than 15 relative to previously published simulations of this type. This enables
dramatic advances in our understanding of the structure and substructure of dark matter in our Galaxy. Our project seeks clues
to the nature of the dark matter and aims to advance strategies for exploring the formation of our Galaxy, for searching for
signals from dark matter annihilation, and for designing experiments for direct detection of dark matter. Here we report on
the status of our calculations carried out on the HLRB-2 thus far, and discuss some of the early results we obtained. Our
results show much better convergence for the properties of dark matter substructures than ever reported in the literature
before. For the first time, we can reliably probe the central dark matter density cusp into a regime where the local logarithmic
slope becomes shallower than−1. We also provide a description of the simulation code GADGET-3 developed specifically for
this project, and highlight the new parallelization techniques we employed to deal with the extremely tightly coupled nature
and high dynamic range of our simulations.

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    ABSTRACT: We use cosmological LCDM numerical simulations to model the evolution of the substructure population in sixteen dark matter haloes with resolutions of up to seven million particles within the virial radius. The combined substructure circular velocity distribution function (VDF) for hosts of 10^11 to 10^14 Msun at redshifts from zero to two or higher has a self-similar shape, is independent of host halo mass and redshift, and follows the relation: dn/dv=(1/8)(v_cmax/v_cmax,host)^-4. Halo to halo variance in the VDF is a factor of roughly two to four. At high redshifts, we find preliminary evidence for fewer large substructure haloes (subhaloes). Specific angular momenta are significantly lower for subhaloes nearer the host halo centre where tidal stripping is more effective. The radial distribution of subhaloes is marginally consistent with the mass profile for r >~ 0.3r_vir, where the possibility of artificial numerical disruption of subhaloes can be most reliably excluded by our convergence study, although a subhalo distribution that is shallower than the mass profile is favoured. Subhalo masses but not circular velocities decrease toward the host centre. Subhalo velocity dispersions hint at a positive velocity bias at small radii. There is a weak bias toward more circular orbits at lower redshift, especially at small radii. We additionally model a cluster in several power law cosmologies of P ~ k^n, and demonstrate that a steeper spectral index, n, results in significantly less substructure.
    Monthly Notices of the Royal Astronomical Society 07/2004; · 5.52 Impact Factor