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The arms in M51 have been triggered by an m=2 gravitational perturbation by the tidal encounter with M52. The arms are "trailing" -- meaning the direction of rotation is with the convex part of the arm leading. Since gas and stars pass through the arms, the gas shocks and forms stars and dust lanes on the inside of the arm -- i.e. on its concave side. On the right is a CO map, which for the most part follows the dust lanes. On the lower left, the CO traces the inside (concave) part of the arm quite clearly.
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