A galactic disk as a two-fluid system - Consequences for the critical stellar velocity dispersion and the formation of condensations in the gas

The consequences of treating a galactic disk as a two-fluid system for the stability of the entire disk and form of the gas in the disk are examined. It is found that the existence of even a small fraction of the total disk surface density in a cold fluid makes it much harder to stabilize the entire two-fluid disk. At the higher effective gas densities resulting from the growth of a two-fluid instability, the gas may become unstable even when it is itself originally stable. The wavelength of a typical gas instability in the inner galaxy is about 400 pc, and it contains about 10 million solar masses of interstellar matter; these instabilities may be identified with clusters of giant molecular clouds. It is suggested that many of the spiral features seen in gas-rich spiral galaxies may be material arms or arm segments resulting from sheared two-fluid gravitational instabilities.