Density waves in the outer disk of spiral galaxies

Several spiral galaxies display a prominent coherent spiral structure in their gaseous outer disk. Here we argue that these observed coherent patterns can be naturally interpreted as the manifestation of the mechanism that excites grand-design spiral structure in the main, star-dominated body of the disk. While a discrete spectrum of global modes is determined by the structure of the main body of the disk, which acts as a resonant cavity, the excitation is expected to be 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. 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, much like ocean waves moving close to the shore. In this context, we will describe some interesting results on the role of finite thickness effects.