Spurious Small-Scale Structure & Discreteness-Driven Relaxation in Cosmological Simulations

TL;DR

This paper investigates the origin of spurious low-mass haloes in cosmological N-body simulations, attributing their formation to discreteness-driven relaxation and proposing methods to mitigate this artifact.

Contribution

It demonstrates that spurious haloes result from discreteness effects in traditional N-body simulations and suggests improved algorithms to reduce this issue.

Findings

Spurious haloes are seeded at shell crossing by local velocity perturbations.

Characteristic separation of spurious haloes matches mean inter-particle distance.

Adaptive gravitational softening can partially suppress discreteness-driven relaxation.

Abstract

There is strong evidence that cosmological N-body simulations dominated by Warm Dark Matter (WDM) contain spurious or unphysical haloes, most readily apparent as regularly spaced low-mass 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 Dark Matter (CDM) models, 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 localised 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 fixed mass ratio, we find that the spatial distribution of the two components show signatures of discreteness-driven relaxation in their spatial distribution on both large and small scales. Adopting a spline kernel gravitational softening that is of order the comoving mean inter-particle 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 solutions for the evolution of dark matter in phase space.