Rotation curves for M31 have been obtained from both optical spectra and 21-cm velocity measurements. Even the earliest optical spectra were useful for understanding the gross rotational parameters of the galaxy, and subsequent optical data have greatly refined the details of what turns out to be a rather complicated rotation curve. The hydrogen gas motions agree, in general, with the optical data, but extend out to greater distances, giving important knowledge about the outer mass distribution (unfortunately, not enough knowledge, however, to answer the questions that are raised there). Very recently, the rotation of the nucleus has been scrutinized with modern techniques, to reveal a rapidly rotating object that may be driven by a massive central black hole.
KeywordsRadio Telescope Rotation Curve Radial Motion Image Tube Systemic Velocity
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