Quasi-stationary spiral structure in galaxies

The hypothesis that the grand design observed in spiral galaxies may be described by the superposition (and interaction) of a small number of spiral modes is reaffirmed. Attention is called to the present capability of simulating galaxies of various morphological types in a controllable manner. What is more, the processes that govern the maintenance and excitation of these modes simulating both normal spirals and barred spirals can be understood by using analytical theories that are closely related to the local dispersion relationship. The powerful 'swing' amplification process is seen as suggesting that the spiral structure might be rapidly varying, on the time scale of an epicyclic period, and not quasi-stationary. However, this perception of swing amplification holds only for wave packets. In the context of steady wave trains or modes, it is possible to show that an exactly equivalent amplification mechanism can be described in terms of a process of wave amplification via stimulation of emission of radiation.