Novel Conservative Methods for Adaptive Force Softening in Collisionless and Multi-Species N-Body Simulations

TL;DR

This paper introduces new conservative adaptive force softening methods for collisionless N-body simulations, improving accuracy, conservation, and physical realism, especially in multi-species and high-dynamic-range scenarios.

Contribution

It derives a generalized energy-momentum conserving framework for adaptive softening, introduces novel softening schemes based on gravitational properties, and implements these in the GIZMO code.

Findings

Tidal softening scheme is physically motivated, Galilean invariant, and conservative.

The new methods reduce artificial disruption and N-body heating.

Implementation in GIZMO demonstrates improved accuracy and efficiency.

Abstract

Modeling self-gravity of collisionless fluids (e.g. ensembles of dark matter, stars, black holes, dust, planetary bodies) in simulations is challenging and requires some force softening. It is often desirable to allow softenings to evolve adaptively, in any high-dynamic range simulation, but this poses unique challenges of consistency, conservation, and accuracy, especially in multi-physics simulations where species with different softening laws may interact. We therefore derive a generalized form of the energy-and-momentum conserving gravitational equations of motion, applicable to arbitrary rules used to determine the force softening, together with consistent associated timestep criteria, interaction terms between species with different softening laws, and arbitrary maximum/minimum softenings. We also derive new methods to maintain better accuracy and conservation when symmetrizing forces between particles. We review and extend previously-discussed adaptive softening schemes based on the local neighbor particle density, and present several new schemes for scaling the softening with properties of the gravitational field, i.e. the potential or acceleration or tidal tensor. We show that the tidal softening scheme not only represents a physically-motivated, translation and Galilean invariant and equivalence-principle respecting (and therefore conservative) method, but imposes negligible timestep or other computational penalties, ensures that pairwise two-body scattering is small compared to smooth background forces, and can resolve outstanding challenges in properly capturing tidal disruption of substructures (minimizing artificial destruction) while also avoiding excessive N-body heating. We make all of this public in the GIZMO code.