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.

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.

- References (12)
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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; 359(4). DOI:10.1111/j.1365-2966.2005.09020.x · 5.11 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We present initial results from ``Via Lactea'', the highest resolution simulation to date of Galactic CDM substructure. It follows the formation of a Milky Way-size halo with Mvir=1.8x10^12 Msun in a WMAP 3-year cosmology, using 234 million particles. Over 10,000 subhalos can be identified at z=0: Their cumulative mass function is well-fit by N(>Msub)= 0.0064 (Msub/Mvir)^(-1) down to Msun=4x10^6 Msun. The total mass fraction in subhalos is 5.3%, while the fraction of surface mass density in substructure within a projected distance of 10 kpc from the halo center is 0.3%. Because of the significant contribution from the smallest resolved subhalos, these fractions have not converged yet. Sub-substructure is apparent in all the larger satellites, and a few dark matter lumps are resolved even in the solar vicinity. The number of dark satellites 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 practically smooth in previous simulations. The neutralino self-annihilation gamma-ray emission from dark matter clumps is approximately constant per subhalo mass decade. Therefore, while in our run the contribution of substructure to the gamma-ray luminosity of the Galactic halo amounts to only 40% of the total spherically-averaged smooth signal, we expect this fraction to grow significantly as resolution is increased further. An all-sky map of the expected annihilation gamma-ray flux reaching a fiducial observer at 8 kpc from the Galactic center shows that at the current resolution a small number of subhalos start to be bright enough to be visible against the background from the smooth density field surrounding the observer.The Astrophysical Journal 12/2006; 657(1). DOI:10.1086/510736 · 5.99 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The results of numerical simulations of nonlinear gravitational clustering in universes dominated by weakly interacting, 'cold' dark matter are presented. The numerical methods used and the way in which initial conditions were generated are described, and the simulations performed are catalogued. The evolution of the fundamental statistical properties of the models is described and their comparability with observation is discussed. Graphical comparisons of these open models with the observed galaxy distribution in a large redshift survey are made. It is concluded that a model with a cosmological density parameter omega equal to one is quite unacceptable if galaxies trace the mass distribution, and that models with omega of roughly two, while better, still do not provide a fully acceptable match with observation. Finally, a situation in which galaxy formation is suppressed except in sufficiently dense regions is modelled which leads to models which can agree with observation quite well even for omega equal to one.The Astrophysical Journal 06/1985; 292. DOI:10.1086/163168 · 5.99 Impact Factor

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