Spiral Structure in Galaxies

Jerry Sellwood

Rutgers University

Recurrent spiral patterns in galaxy disks redistribute angular momentum, increase random motions, cause radial mixing, smooth rotation curves, amplify the magnetic field, etc. I present a general assessment of our understanding of the origin of spiral patterns in galaxies. The picture that may ultimately emerge is one in which the spiral patterns are true instabilities of a non-smooth disk, and the saturation and decay of one instability seeds the growth of another. Future ground- and space-based surveys to measure the kinematics and chemistry of stars across the Milky Way will yield data that could confirm or refute this idea.