Stellar kinematicsare hard to derive for Seyfert nuclei, much less quasars, because the starlight is swamped by the nonstellar light from the nucleus. It's easier for radio galaxies (like, for example, M87). On the theory that the compact radio source Sagittarius A*is some kind of weak AGN, studies of kinematics near the Milky Way's nuclear become relevant. A real breathrough has been the newly acquired ability to measure proper motions of stars within the central parsec over a few years. Ghez et al. (2000 Nature 407, 349) show arcs for these motions which suggest a central mass of 2.6 × 106solar masses. The great thing about proper-motion studies is that the results only improve with time. Current datashow substantial parts of stellar orbits here, making it clear thet they are Keplerian about a mass smaller than about 10 AU in radius, and making this by far the best-attested massive black hole. (Link to animated GIF ofstellar orbits) Gravitational redshift of X-ray spectral features.
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