In spiral galaxies, the pitch angle, α, of the spiral arms is often proposed as a discriminator between theories for the formation of the spiral structure. In Lin–Shu density wave theory, α stays constant in time, being simply a property of the underlying galaxy. In other theories (e.g. tidal interaction, and self-gravity), it is expected that the arms wind up in time, so that to a first approximation |$\cot \alpha \propto t$|⁠. For these theories, it would be expected that a sample of galaxies observed at random times should show a uniform distribution of |$\cot \alpha$|⁠. We show that a recent set of measurements of spiral pitch angles (Yu & Ho) is broadly consistent with this expectation.

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