Spurious haloes and discreteness-driven relaxation in cosmological simulations

C. Power, A.S.G. Robotham, D. Obreschkow, A. Hobbs, G. F. Lewis

Research output: Contribution to journalArticle

15 Citations (Scopus)
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Abstract

There is strong evidence that cosmologicalN-body simulations dominated bywarm darkmatter (WDM) contain spurious or unphysical haloes, most readily apparent as regularly spaced lowmass haloes strung along filaments. We show that spurious haloes are a feature of traditional N-body simulations of cosmological structure formation models, including WDM and cold darkmattermodels, in which gravitational collapse proceeds in an initially anisotropic fashion, and arises naturally as a consequence of discreteness-driven relaxation. We demonstrate this using controlled N-body simulations of plane-symmetric collapse and show that spurious haloes are seeded at shell crossing by localized velocity perturbations induced by the discrete nature of the density field, and that their characteristic separation should be approximately the mean inter-particle separation of the N-body simulation, which is fixed by the mass resolution within the volume. Using cosmological N-body simulations in which particles are split into two collisionless components of fixedmass ratio, we find that the spatial distribution of the two components show signatures of discreteness-driven relaxation on both large and small scales. Adopting a spline kernel gravitational softening that is of order the comoving mean interparticle separation helps to suppress the effect of discreteness-driven relaxation, but cannot eliminate it completely. These results provide further motivation for recent developments of new algorithms, which include, for example, revisions of the traditional N-body approach by means of spatially adaptive anistropric gravitational softenings or explicit solution of the evolution of dark matter in phase space.

Original languageEnglish
Article numberstw1644
Pages (from-to)474-489
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Volume462
Issue number1
DOIs
Publication statusPublished - 11 Oct 2016

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