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Title:
The Proper Motion of Sagittarius A*. II. The Mass of Sagittarius A*
Authors:
Reid, M. J.; Brunthaler, A.
Affiliation:
AA(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; ), AB(Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany; and Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, Netherlands; )
Publication:
The Astrophysical Journal, Volume 616, Issue 2, pp. 872-884. (ApJ Homepage)
Publication Date:
12/2004
Origin:
UCP
ApJ Keywords:
Astrometry, Black Hole Physics, Galaxy: Center, Galaxy: Fundamental Parameters, Galaxy: Structure
DOI:
10.1086/424960
Bibliographic Code:
2004ApJ...616..872R

Abstract

We report measurements with the Very Long Baseline Array (VLBA) of the position of Sgr A* with respect to two extragalactic radio sources over a period of 8 yr. The apparent proper motion of Sgr A* relative to J1745-283 is 6.379+/-0.024 mas yr-1 along a position angle of 209.60d+/-0.18d, almost entirely in the plane of the Galaxy. The effects of the orbit of the Sun around the Galactic center can account for this motion, and the residual proper motion of Sgr A* perpendicular to the plane of the Galaxy is -0.4+/-0.9 km s-1. A maximum likelihood analysis of the motion expected for a massive object within the observed Galactic center stellar cluster indicates that Sgr A* contains more than about 10% of the ~4×106 Msolar deduced from stellar orbits. The intrinsic size of Sgr A*, as measured by several investigators, is less than 1 AU, and the implied mass density of ~1022 Msolar pc-3 is within about 3 orders of magnitude of a comparable supermassive black hole within its Schwarzschild radius. Our observations provide the first direct evidence that a compact radiative source at the center of a galaxy contains of order 106 Msolar and provides overwhelming evidence that it is in the form of a supermassive black hole. Finally, the existence of ``intermediate-mass'' black holes more massive than ~104 Msolar between roughly 103 and 105 AU from Sgr A* is excluded.

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