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          Institute: MPI für Astrophysik     Collection: Cosmology     Display Documents

ID: 10397.0, MPI für Astrophysik / Cosmology
The inner structure of ΛCDM haloes - I. A numerical convergence study
Authors:Power, C.; Navarro, J. F.; Jenkins, A.; Frenk, C. S.; White, S. D. M.; Springel, V.; Stadel, J.; Quinn, T.
Date of Publication (YYYY-MM-DD):2003-01-01
Title of Journal:Monthly Notices of the Royal Astronomical Society
Journal Abbrev.:Mon. Not. Roy. Astron. Soc.
Issue / Number:1
Start Page:14
End Page:34
Review Status:Peer-review
Audience:Experts Only
Abstract / Description:We present a comprehensive set of convergence tests which explore the role of various numerical parameters on the equilibrium structure of a simulated dark matter halo. We report on results obtained with two independent, state-of-the- art, multi-stepping, parallel N-body codes: pkdgrav and gadget. We find that convergent mass profiles can be obtained for suitable choices of the gravitational softening, time-step, force accuracy, initial redshift, and particle number. For softenings chosen so that particle discreteness effects are negligible, convergence in the circular velocity is obtained at radii where the following conditions are satisfied: (i) the time-step is much shorter than the local orbital time-scale; (ii) accelerations do not exceed a characteristic acceleration imprinted by the gravitational softening; and (iii) enough particles are enclosed so that the collisional relaxation time- scale is longer than the age of the Universe. Convergence also requires sufficiently high initial redshift and accurate force computations. Poor spatial, time, or force resolution leads generally to systems with artificially low central density, but may also result in the formation of artificially dense central cusps. We have explored several adaptive time-stepping choices and we have obtained the best results when individual time- steps are chosen according to the local acceleration and the gravitational softening (Deltat(i) proportional to (epsilon/a(i) )(1/2)), although further experimentation may yield better and more efficient criteria. The most stringent requirement for convergence is typically that imposed on the particle number by the collisional relaxation criterion. This implies that, in order to estimate accurate circular velocities at radii where the density contrast may reach similar to10(6), the region must enclose of the order of 3000 particles (or more than a few times 10(6) within the virial radius). Applying these criteria to a galaxy-sized LambdaCDM halo, we find that the spherically averaged density profile becomes progressively shallower from the virial radius inwards, reaching a logarithmic slope shallower than -1.2 at the innermost resolved point, similar to0.005 r (200), with little evidence for convergence to a power-law behaviour in the inner regions.
Free Keywords:gravitation; cosmology : theory; dark matter
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astrophysik
External Affiliations:Univ Durham, Dept Phys, South Rd, Durham DH1 3LE, England; Univ Durham, Dept Phys, Durham DH1 3LE, England; Univ Victoria, Dept Phys & Astron, Victoria, BC V8P 1A1, Canada; Max Planck Inst Astrophys, D-85740 Munich, Germany; Univ Washington, Dept Astron, Seattle, WA 98195 USA
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