NYU undergraduate Bob Ma accomplished something nice today. He measured the distances of a set of HII regions from the center of M51 (literally with a ruler, working on the HST image), and also their angles relative to a reference angle (ie, their polar coordinates). What did he find? That the angle depends linearly on radius. This is exactly what you expect if the spiral arm pattern in M51 was begun as an m=2 distortion that wrapped up under the action of a flat rotation curve. What raised the m=2 distortion? The interaction with the companion, of course. The slope of the line on the radius–angle plot gives you the time since the raising of the distortion. This is all part of my evil plan to demonstrate that spiral patterns are transient, and to identify their causes, individually, in all sufficiently well-observed galaxies.
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