Spiral structure as a recurrent instability

A snapshot view of galaxies makes it difficult to decide whether spirals are long-lived and slowly evolving or recurrent transient structures. The behavior of N-body simulations has long suggested the latter, but the short-lived waves in the simulations must be understood before it can be argued with confidence that galaxies behave in a similar manner. This paper describes rapidly growing groove instabilities that give rise to large-scale spiral patterns in simulations. Resonant scattering by one wave causes a new instability elsewhere in the disk, leading to a recurrent cycle that promise to account for the rapidly evolving patterns in the simulations. There seems no reason why this instability cycle should not also occur in galaxies. 16 refs