Gravitational Softening and Adaptive Mass Resolution

TL;DR

This paper analyzes gravitational softening in N-body simulations, providing new formulas for force calculations between particles of different softening lengths and profiles, improving accuracy and flexibility.

Contribution

It introduces closed-form expressions for pairwise gravitational forces with arbitrary softening scales and profiles, clarifying the consistency of Plummer softening and extending to various particle types.

Findings

Plummer softening is consistent with diffuse particle interpretation.

New formulas for forces between particles with different softening lengths.

Enhanced modeling capabilities for particles like stars or black holes.

Abstract

Pairwise forces between particles in cosmological N-body simulations are generally softened to avoid hard collisions. Physically, this softening corresponds to treating the particles as diffuse clouds rather than point masses. For particles of unequal mass (and hence unequal softening length), computing the softened force involves a nontrivial double integral over the volumes of the two particles. We show that Plummer force softening is consistent with this interpretation of softening while spline softening is not. We provide closed-form expressions and numerical implementation for pairwise gravitational force laws for pairs of particles of general softening scales $\epsilon_1$ and $\epsilon_2$ assuming the commonly used cloud profiles: NGP, CIC, TSC, and PQS. Similarly, we generalize Plummer force law into pairs of particles of general softenings. We relate our expressions to the gaussian, Plummer and spline force softenings known from literature. Our expressions allow possible inclusions of pointlike particles such as stars or supermassive black holes.