Tidal adaptive softening and artificial fragmentation in cosmological simulations

TL;DR

This paper investigates whether tidal adaptive softening can reduce artificial fragmentation in cosmological N-body simulations, finding it improves force accuracy but does not significantly decrease spurious halo formation, especially with optimized initial conditions.

Contribution

It introduces and tests a tidal adaptive softening method in cosmological simulations, analyzing its effects on artificial fragmentation and halo formation compared to fixed softening.

Findings

Tidal adaptive softening improves force accuracy in idealised filament simulations.

It does not significantly reduce spurious haloes in cosmological simulations.

Initial conditions at earlier redshifts lead to less spherical Lagrangian volumes for halo formation.

Abstract

Traditional N-body methods introduce localised perturbations in the gravitational forces governing their evolution. These perturbations lead to an artificial fragmentation in the filamentary network of the Large Scale Structure, often referred to as "beads-on-a-string." This issue is particularly apparent in cosmologies with a suppression of the matter power spectrum at small spatial scales, such as warm dark matter models, where the perturbations induced by the N-body discretisation dominate the cosmological power at the suppressed scales. Initial conditions based on third-order Lagrangian perturbation theory, which allow for a late-starting redshift, have been shown to minimise numerical errors contributing to such artefacts. In this work, we investigate whether the additional use of a spatially adaptive softening for dark matter particles, based on the gravitational tidal field, can reduce the severity of artificial fragmentation. Tidal adaptive softening significantly improves force accuracy in idealised filamentary collapse simulations over a fixed softening approach. However, it does not substantially reduce spurious haloes in cosmological simulations when paired with such optimised initial conditions. Nevertheless, tidal adaptive softening induces a shift in halo formation times in warm dark matter simulations compared to a fixed softening counterpart, an effect not seen in cold dark matter simulations. Furthermore, initialising the initial conditions at an earlier redshift generally results in z=0 haloes forming from Lagrangian volumes with lower average sphericity. This sphericity difference could impact post-processing algorithms identifying spurious objects based on Lagrangian volume morphology. We propose potential strategies for reducing spurious haloes without abandoning current N-body methods.