Spiral Instabilities in N-body Simulations: Emergence from noise II

TL;DR

This paper confirms that noise-induced spiral instabilities in N-body simulations of stellar discs grow over time due to non-linear effects, and this phenomenon is robust across different numerical schemes.

Contribution

It demonstrates that spiral instabilities from shot noise are genuine and not artifacts of a specific numerical method, confirming earlier findings with an alternative grid geometry.

Findings

Disturbances grow to large amplitudes over time.

Instabilities are caused by non-linear scattering from shot noise.

Results are consistent across different numerical schemes.

Abstract

An earlier paper presented the potentially significant discovery that disturbances in simplified simulations of a stellar disc model that was predicted to be stable in linear theory grew to large amplitude over a long period of time. The ultimate appearance of true instabilities was attributed to non-linear scattering by a succession of collective waves excited by shot noise from the finite number of particles. The paper concluded that no finite number of particles, however large, could mimic a smooth disc. As this surprising finding has been challenged as an artifact of the numerical scheme employed, we here present a new calculation of the same model using a different grid geometry that confirms the original behaviour.