Prominent spiral arms in the gaseous outer galaxy disks
  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.
