Non-axisymmetric disturbances in galactic discs

Summary

Introduction

In two contexts, this talk revisited an old idea – that any concentrated mass orbiting within a shearing disc tends to create an inclined and sometimes quite intense wavelike wake amidst the material that flows past it. More exactly, as actually delivered, this talk began with a fairly lengthy review of such swing-amplified wake-making in the gorgeous global setting of Zang's V = const disc, and only then did it proceed to my two main contexts.

Polarisation in the shearing sheet

One context was the idealized shearing sheet composed of thousands of identical softened mass points, with which I have been conducting a long series of numerical experiments in recent years. As I illustrated with slides and even with a homemade (!) movie, the typical impression there is one of an ever-changing kaleidoscope of “spiral” features, very much suggesting some sort of a recurrent strong instability in a system that really ought not to have any. Appearances aside, it turns in fact that those features are no instabilities. Instead, they are logically just superpositions of the separate wakes of the many random particles, each of which in this sense keeps acting simultaneously as both an aggressor and a victim. It is the collection of these wakes that forever keeps shifting in location and appearance as the individual particles constantly drift in this shear flow, but each particle in turn always carries along a wake of its own.