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Paper: The Proper Motion of Sgr A*
Volume: 340, Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA
Page: 253
Authors: Reid, M.J.; Brunthaler, A.
Abstract: We have now been measuring the position of Sgr A*, the candidate super-massive black hole at the center of the Galaxy, with the VLBA for about 8 years. Sgr A* appears to move almost entirely along the Galactic Plane at a rate of 6.37±0.02 mas yr−1. For a distance to the Galactic Center of 8.0±0.5 kpc, this translates to 241 ± 15 km s−1 , consistent with that expected for a stationary object observed from the Sun as it orbits the Galactic Center. The motion of Sgr A* out of the plane of the Galaxy, after removing the 7 km s−1 motion of the Sun in that direction, is less than about 2 km s−1 . Combining stellar orbital information (measured in the infrared) with the upper limit of 2 km s−1 for the intrinsic proper motion of Sgr A* (perpendicular to the Galactic plane), places a lower limit on the mass of Sgr A* of 2 × 106 Msolar. Thus, most of the mass sensed by stellar orbits is tied to the compact radio source Sgr A*, whose size is less than 1 AU, yielding the strongest case ever for a SMBH. This also argues against "exotic" forms of mass, postulated to explain the extreme mass concentration at the Galactic Center.
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