The diversity and similarity of simulated cold dark matter haloes

Department of Astronomy, University of Massachusetts, Amherst, MA 01003-9305, USA
Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 02/2010; 402(1):21 - 34. DOI: 10.1111/j.1365-2966.2009.15878.x


We study the mass, velocity dispersion and anisotropy profiles of Λ cold dark matter (ΛCDM) haloes using a suite of N-body simulations of unprecedented numerical resolution. The Aquarius Project follows the formation of six different galaxy-sized haloes simulated several times at varying numerical resolution, allowing numerical convergence to be assessed directly. The highest resolution simulation represents a single dark matter halo using 4.4 billion particles, of which 1.1 billion end up within the virial radius. Our analysis confirms a number of results claimed by earlier work, and clarifies a few issues where conflicting claims may be found in the recent literature. The mass profile of ΛCDM haloes deviates slightly but systematically from the form proposed by Navarro, Frenk & White. The spherically averaged density profile becomes progressively shallower inwards and, at the innermost resolved radius, the logarithmic slope is γ≡− d ln ρ/d ln r≲ 1. Asymptotic inner slopes as steep as the recently claimed ρ∝r−1.2 are clearly ruled out. The radial dependence of γ is well approximated by a power law, γ∝rα (the Einasto profile). The shape parameter, α, varies slightly but significantly from halo to halo, implying that the mass profiles of ΛCDM haloes are not strictly universal: different haloes cannot, in general, be rescaled to look identical. Departures from similarity are also seen in velocity dispersion profiles and correlate with those in density profiles so as to preserve a power-law form for the spherically averaged pseudo-phase-space density, ρ/σ3∝r−1.875. The index here is identical to that of Bertschinger's similarity solution for self-similar infall on to a point mass from an otherwise uniform Einstein–de Sitter universe. The origin of this striking behaviour is unclear, but its robustness suggests that it reflects a fundamental structural property of ΛCDM haloes. Our conclusions are reliable down to radii below 0.4 per cent of the virial radius, providing well-defined predictions for halo structure when baryonic effects are neglected, and thus an instructive theoretical template against which the modifications induced by the baryonic components of real galaxies can be judged.

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Available from: Aaron D. Ludlow, Jan 05, 2014
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    • "−0 . 39 , when excluding the inner 40 kpc / h . While the total density profile is in agreement with a NFW profile ( α = 1 ) , the inner DM profile is shallower than simulations . Even considering recent simulations ( Stadel et al . 2009 ; Navarro et al . 2010 ) the minimum slope obtained , α −0 . 8 at 120 pc , ( Stadel et al . 2009 ) is larger than the results of observations , and the scatter in the slope from cluster to cluster is much larger than what found in simulations ( see figure 2 , gray points ) . If some part of the scatter can be explained , as previously reported , in terms of t"
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    • "The DM mass distribution within halos is well described by a near-universal density profile, the so-called NFW profile [30], which has the form of a double-power-law with the logarithmic slope c d log q/d log r transitioning at the scale radius r s from c = À3 at large radii to c = À1 in the center. More recent higher resolution simulations, however, have found a central slope shallower than c = À1, indicating that the density profile may be better described by a functional form with a central slope gradually flattening to c = 0, e.g. the Einasto profile [31] [32]. "
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