Prominent spiral arms in the gaseous outer galaxy disks

TL;DR

This paper explains the prominent spiral arms in the gaseous outer disks of galaxies as a natural consequence of density waves driven by angular momentum transport, providing a new perspective on galaxy structure and dark matter distribution.

Contribution

It introduces a theory that outer spiral arms result from density waves excited by angular momentum transport, linking observed structures to disk and dark matter properties.

Findings

Amplitude profiles fit with linear wave theory constrain disk density.

Outer spiral arms are driven by inward angular momentum transport.

Results inform disk-halo decomposition in galaxy rotation curves.

Abstract

Context: Several spiral galaxies, as beautifully exhibited by the case of NGC 6946, display a prominent large-scale spiral structure in their gaseous outer disk. Such structure is often thought to pose a dynamical puzzle, because grand-design spiral structure is traditionally interpreted as the result of density waves carried mostly in the stellar disk. Aims. Here we argue that the outer spiral arms in the cold gas outside the bright optical disk actually have a natural interpretation as the manifestation of the mechanism that excites grand-design spiral structure in the main, star-dominated body of the disk: the excitation is driven by angular momentum transport to the outer regions, through trailing density waves outside the corotation circle that can penetrate beyond the Outer Lindblad Resonance in the gaseous component of the disk. Methods: Because of conservation of the density wave action, these outgoing waves are likely to become more prominent in the outer disk and eventually reach non-linear amplitudes. To calculate the desired amplitude profiles, we make use of the theory of dispersive waves. Results: If the conditions beyond the optical radius allow for an approximate treatment in terms of a linear theory, we show that fitting the observed amplitude profiles leads to a quantitative test on the density of the disk material and thus on the dark matter distribution in the outer parts of the galaxy. Conclusions: This study is thus of interest to the general problem of the disk-halo decomposition of rotation curves.