We examine the development of a transient spiral arm in a disk galaxy made up of both gas and stars. To this end we have performed numerical simulations in a shearing sheet (basically a rectangular patch of a disc) that contains gas in the form of clouds behaving like Brahics (1977) sticky particles, and stars that appear as a background continuum providing the perturbation forces. These are computed from the theory of swing amplification, using Fuchs (1991) work. We describe the evolution of our model under a single and under recurring swing amplification events, discerning three phases. Furthermore, we give an interpretation of this evolution in terms of a variation of the epicyclic frequency with the distance to the wave crest. We also assess the importance of self gravity in the gas for our results.
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