Spiral density waves in the outer galactic gaseous discs

TL;DR

This paper investigates how large-scale spiral density waves propagate into the outer gaseous discs of galaxies, revealing their structure, amplitude growth, and instability, which helps understand galaxy dynamics and dark matter distribution.

Contribution

It demonstrates through hydrodynamical simulations that spiral waves can extend far beyond the optical disc, matching linear theory at low amplitudes and revealing nonlinear and unstable behavior at larger radii.

Findings

Spiral arms can extend up to 25 disc scale-lengths in simulations.

Linear theory matches low-amplitude spiral structures.

Spiral amplitude increases rapidly with radius and becomes unstable.

Abstract

Deep HI observations of the outer parts of disc galaxies demonstrate the frequent presence of extended, well-developed spiral arms far beyond the optical radius. To understand the nature and the origin of such outer spiral structure, we investigate the propagation in the outer gaseous disc of large-scale spiral waves excited in the bright optical disc. Using hydrodynamical simulations, we show that non-axisymmetric density waves, penetrating in the gas through the outer Lindblad resonance, can exhibit relatively regular spiral structures outside the bright optical stellar disc. For low-amplitude structures, the results of numerical simulations match the predictions of a simple WKB linear theory. The amplitude of spiral structure increases rapidly with radius. Beyond $\approx 2$ optical radii, spirals become nonlinear (the linear theory becomes quantitatively and qualitatively inadequate) and unstable to Kelvin-Helmholtz instability. In numerical simulations, in models for which gas is available very far out, spiral arms can extend out to 25 disc scale-lengths. A comparison between the properties of the models we have investigated and the observed properties of individual galaxies may shed light into the problem of the amount and distribution of dark matter in the outer halo.