Swinging Spiral Arms

TL;DR

This paper investigates transient spiral structures in galactic discs, showing that they grow through swing amplification driven by a central source, with the structure winding up over time and leaving behind an exponential density profile.

Contribution

It introduces a model of transient spiral growth via swing amplification in an isothermal galactic disc, emphasizing the role of a central source and the winding process.

Findings

Spiral structures grow due to swing amplification from a central source.

Transient spirals wind up over time, leaving exponential density profiles.

No instability grows everywhere from infinitesimal disturbances.

Abstract

We study transient spiral structures in an isothermal, thin, galactic disc. We find no instability that can grow everywhere from infinitesimal disturbances, but spiral structure does grow in the disc due to an arbitrarily strong, asymmetric, central source. An initially finite spiral undergoes transient swing amplification as it is gradually wound-up by differential rotation. An independent sequence in negative time describes a leading spiral swinging to a trailing spiral. The dynamical coupling is established between the swinging potential and the arm particles, by ensuring that this potential constrains a locally rotating distribution function centred on the arms. This swing amplification propagates in radius at the constant rotational speed of the disc, and leaves behind an exponential density decline in space and time.