The proper motion of Sagittarius A*. III. The case for a supermassive black hole.
REID M.J. and BRUNTHALER A.
Abstract (from CDS):
We report measurements with the Very Long Baseline Array of the proper motion of Sgr A* relative to two extragalactic radio sources spanning 18 yr. The apparent motion of Sgr A* is -6.411 ± 0.008 mas yr–1 along the Galactic plane and -0.219 ± 0.007 mas yr–1 toward the North Galactic Pole. This apparent motion can almost entirely be attributed to the effects of the Sun's orbit about the Galactic center. Removing these effects yields residuals of -0.58 ± 2.23 km s–1 in the direction of Galactic rotation and -0.85 ± 0.75 km s–1 toward the North Galactic Pole. A maximum-likelihood analysis of the motion, both in the Galactic plane and perpendicular to it, expected for a massive object within the Galactic center stellar cluster indicates that the radiative source, Sgr A*, contains more than about 25% of the gravitational mass of 4 x 106 M☉ deduced from stellar orbits. The intrinsic size of Sgr A* is comparable to its Schwarzschild radius, and the implied mass density of ≳4×1023M☉ pc–3 is very close to that expected for a black hole, providing overwhelming evidence that it is indeed a supermassive black hole. Finally, "intermediate mass" black holes more massive than ≃3 x 104 M☉ between approximately 0.003 and 0.1 pc from Sgr A* are excluded.