Spiral eigenmodes triggered by grooves in the phase space of disc galaxies

TL;DR

This paper demonstrates that narrow grooves in the phase space of disc galaxies can induce new, rapidly growing spiral eigenmodes, suggesting recurrent spiral structures driven by phase space alterations.

Contribution

It introduces a linear perturbation theory analysis showing how phase-space grooves generate unique, unstable eigenmodes in disc galaxies, without requiring non-linear processes.

Findings

Grooves create new, vigorous eigenmodes in disc galaxies.

Narrow phase-space grooves can be sources of rapidly growing spiral patterns.

Recurrent spiral structures may result from successive mode destabilizations.

Abstract

We use linear perturbation theory to investigate how a groove in the phase space of a disc galaxy changes the stellar disc's stability properties. Such a groove is a narrow trough around a fixed angular momentum from which most stars have been removed, rendering part of the disc unresponsive to spiral waves. We find that a groove can dramatically alter a disc's eigenmode spectrum by giving rise to a set of vigorously growing eigenmodes. These eigenmodes are particular to the grooved disc and are absent from the original ungrooved disc's mode spectrum. We discuss the properties and possible origin of the different families of new modes. By the very nature of our technique, we prove that a narrow phase-space groove can be a source of rapidly growing spiral patterns that are true eigenmodes of the grooved disc and that no non-linear processes need to be invoked to explain their presence in N-body simulations of disc galaxies. Our results lend support to the idea that spiral structure can be a recurrent phenomenon, in which one generation of spiral modes alters a disc galaxy's phase space in such a way that a following generation of modes is destabilized.