Spurious haloes and discretenessdriven relaxation in cosmological simulations
Abstract
There is strong evidence that cosmological Nbody simulations dominated by warm dark matter (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 Nbody simulations of cosmological structure formation models, including WDM and cold dark matter models, in which gravitational collapse proceeds in an initially anisotropic fashion, and arises naturally as a consequence of discretenessdriven relaxation. We demonstrate this using controlled Nbody simulations of planesymmetric 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 interparticle separation of the Nbody simulation, which is fixed by the mass resolution within the volume. Using cosmological Nbody simulations in which particles are split into two collisionless components of fixed mass ratio, we find that the spatial distribution of the two components show signatures of discretenessdriven 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 discretenessdriven 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 Nbody approach by means of spatially adaptive anistropric gravitational softenings or explicit solution of the evolution of dark matter in phase space.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 October 2016
 DOI:
 10.1093/mnras/stw1644
 arXiv:
 arXiv:1606.02038
 Bibcode:
 2016MNRAS.462..474P
 Keywords:

 methods: numerical;
 galaxies: formation;
 galaxies: haloes;
 cosmology: theory;
 dark matter;
 largescale structure of Universe;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 17 pages, 16 figures, to appear in Monthly Notices of the Royal Astronomical Society