# Spiral instabilities: Mechanism for recurrence

## TL;DR

This paper proposes that recurrent spiral patterns in stellar discs are caused by self-excited instabilities, specifically groove-type instabilities triggered by scattering at Lindblad resonances, explaining persistent spiral activity.

## Contribution

It demonstrates that scattering at Lindblad resonances seeds new instabilities, providing a comprehensive mechanism for recurrent spiral patterns in stellar discs.

## Key findings

- Spiral patterns result from superposition of transient modes.
- Resonance scattering at Lindblad resonances triggers new instabilities.
- The mechanism explains persistent spiral activity in galaxies.

## Abstract

We argue that self-excited instabilities are the cause of spiral patterns in simulations of unperturbed stellar discs. In previous papers, we have found that spiral patterns were caused by a few concurrent waves, which we claimed were modes. The superposition of a few steadily rotating waves inevitably causes the appearance of the disc to change continuously, and creates the kind of shearing spiral patterns that have been widely reported. Although we have found that individual modes last for relatively few rotations, spiral activity persists because fresh instabilities appear, which we suspected were excited by the changes to the disc caused by previous disturbances. Here we confirm our suspicion by demonstrating that scattering at either of the Lindblad resonances seeds a new groove-type instability. With this logical gap closed, our understanding of the behaviour in the simulations is almost complete. We believe that our robust mechanism is a major cause of spiral patterns in the old stellar discs of galaxies, including the Milky Way where we have previously reported evidence for resonance scattering in the recently released Gaia data.