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6. SPIRAL GALAXIES

The most convincing evidence for DM in galaxies is provided by the rotation curves of spirals. The rotation velocity of many spirals stays roughly constant out to large radii rather than declining, as would be expected if the visible stars provided all the gravitating mass. This phenomenon became apparent through optical rotation curves (e.g. Rubin, Ford and Thonnard 1980) and, more dramatically, from extended HI rotation curves (Bosma 1978). HI observations have the advantage that the gas disk extends much further than the optical disk, so that it is more difficult to contrive ways to make DM unnecessary. These early results are summarized by Faber and Gallagher (1979).