Global spiral modes in stellar disks containing gas

The authors present a linear study of the role of gas on global spiral modes in stellar disks using a simple two-fluid model. The local destabilizing role of a cold component has been known for many years and calculated by various authors. The authors confirm the effectiveness of a small amount of gas in fueling spiral instabilities with a response which is primarily at short wavelengths by constructing a synthetic diagram which summarizes the local stability analysis. Then they proceed to consider global spiral modes in two-component systems with applications to regimes of astrophysical interest that are expected to be associated with normal spiral structure. Moderate growth spiral modes are found that are very close in structure to those of one-component systems, but for substantially higher values of the stability parameter Q. A simple model of self-regulation is also presented