Spiral instabilities in N-body simulations

TL;DR

This paper reviews how recurrent spiral patterns in N-body galaxy simulations arise from a cycle of instabilities triggered by particle deficiencies, leading to large waves that cause resonant scattering and subsequent instabilities.

Contribution

It provides evidence that recurrent spiral patterns are driven by a cycle of instabilities initiated by particle deficiencies and resonant scattering in N-body simulations.

Findings

Spiral patterns result from a recurrent instability cycle.

Instabilities are triggered by particle deficiencies at specific angular momenta.

Large-amplitude waves cause resonant scattering, leading to new instabilities.

Abstract

N-body simulations of disc galaxies that display recurrent transient spiral patterns are comparatively easy to construct, but are harder to understand. In this paper, I summarise the evidence from such experiments that the spiral patterns result from a recurrent spiral instability cycle. Each wave starts as rapidly growing, small-amplitude instability caused by a deficiency of particles at a particular angular momentum. The resulting large-amplitude wave creates, through resonant scattering, the conditions needed to precipitate a new instability.