Prominent spiral arms in the gaseous outer galaxy disks

摘要

Context. Several spiral galaxies, as beautifully exhibitedby the case of NGC 6946, display a prominent large-scale spiralstructure in their gaseous outer disk. Such structure is often thoughtto pose a dynamical puzzle, because grand-design spiral structure istraditionally interpreted as the result of density waves carried mostlyin the stellar disk. Aims. Here we argue that the outer spiral arms in the cold gasoutside the bright optical disk actually have a natural interpretationas the manifestation of the mechanism that excites grand-design spiralstructure in the main, star-dominated body of the disk: the excitationis driven by angular momentum transport to the outer regions throughtrailing density waves outside the corotation circle that can penetratebeyond the Outer Lindblad Resonance in the gaseous component of thedisk. Methods. Because of conservation of the density wave action,these outgoing waves are likely to become more prominent in the outerdisk and eventually reach nonlinear amplitudes. To calculate thedesired amplitude profiles, we make use of the theory of dispersivewaves. Results. If the conditions beyond the optical radius allow foran approximate treatment in terms of a linear theory, we show thatfitting the observed amplitude profiles leads to a quantitative test onthe density of the disk material and thus on the dark matterdistribution 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. Key words: galaxies: spiral - galaxies: structure - galaxies: halos - galaxies: kinematics anddynamics