The Lifetimes of Spiral Patterns in Disc Galaxies

TL;DR

This paper reviews evidence on the lifetimes of spiral patterns in disc galaxies, arguing that transient, short-lived spirals are more consistent with observations and simulations than long-lived, steady patterns.

Contribution

It provides a comprehensive review and simulation analysis supporting the transient spiral hypothesis over the quasi-steady model.

Findings

Simulations show all spiral patterns are short-lived.

Velocity distribution in the solar neighbourhood supports transient spirals.

External galaxy observations are inconclusive.

Abstract

The rate of internally-driven evolution of galaxy discs is strongly affected by the lifetimes of the spiral patterns they support. Evolution is much faster if the spiral patterns are recurrent short-lived transients rather than long-lived, quasi-steady features. As rival theories are still advocated based on these two distinct hypotheses, I review the evidence that bears on the question of the lifetimes of spiral patterns in galaxies. Observational evidence from external galaxies is frustratingly inconclusive, but the velocity distribution in the solar neighbourhood is more consistent with the transient picture. I present simulations of galaxy models that have been proposed to support quasi-steady, two-arm spiral modes that in fact evolve quickly due to multi-arm instabilities. I also show that all simulations to date manifest short-lived patterns, despite claims to the contrary. Thus the transient hypothesis is favoured by both numerical results and the velocity distribution in the solar neighbourhood.