Structure and evolution of long-lived spiral patterns in galaxies

The authors use N-body simulations to study the non-linear evolution of spontaneous and tidally induced spiral patterns in galaxies on time-scales of several rotation periods of the disc. The authors find that the patterns can sometimes survive with an almost constant amplitude for five revolutions or more, and tend to be regenerated after disappearing temporarily. The pattern velocity is such that the corotation radius is where the self-gravity as measured by the swing amplification of the m = 2 component is strongest, and the amplitude of the pattern is larger when swing amplification is stronger. The shape of the spirals is independent of the origin of the pattern, and corresponds to a critical wave number. The pattern survives longer if the welf-gravity of the disc is strong. If self-gravity is weak, the dominant tidal effect is a kinematic spiral at the inter Lindblad resonance.