Particle number dependence in the non-linear evolution of N-body self-gravitating systems

TL;DR

This paper investigates how the number of particles in N-body simulations affects the convergence of results in self-gravitating systems, revealing that convergence depends heavily on initial conditions and fluctuation stability.

Contribution

It demonstrates that convergence in N-body simulations varies significantly with initial conditions and highlights the impact of discretization fluctuations on system evolution.

Findings

Convergence can be achieved with N ~ 10^3 for some initial conditions within a few dynamical times.

In other cases, even N ~ 10^5 does not guarantee convergence.

Fluctuation stability influences the evolution and potential discrepancies in cosmological dark matter simulations.

Abstract

Simulations of purely self-gravitating N-body systems are often used in astrophysics and cosmology to study the collisionless limit of such systems. Their results for macroscopic quantities should then converge well for sufficiently large N. Using a study of the evolution from a simple space of spherical initial conditions - including a region characterised by so-called "radial orbit instability" - we illustrate that the values of N at which such convergence is obtained can vary enormously. In the family of initial conditions we study, good convergence can be obtained up to a few dynamical times with N $ \sim 10^3$ - just large enough to suppress two body relaxation - for certain initial conditions, while in other cases such convergence is not attained at this time even in our largest simulations with N $\sim 10^5$. The qualitative difference is due to the stability properties of fluctuations introduced by the N-body discretisation, of which the initial amplitude depends on N. We discuss briefly why the crucial role which such fluctuations can potentially play in the evolution of the N-body system could, in particular, constitute a serious problem in cosmological simulations of dark matter.