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M.J. Reid (SAO), A.C.S. Readhead (Caltech), R.C. Vermeulen (NFRA), R.N. Treuhaft (JPL)
We observed Sgr A* and two extragalactic radio sources with the VLBA over a period of two years and measured relative positions with an accuracy approaching 0.1 mas. The apparent proper motion of Sgr A* is 5.90 +/-- 0.4 mas/y, 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. While it takes about 200 My for the Sun to orbit the Galactic Center, this motion can be detected in less than one month's observing with the VLBA!
Assuming that Sgr A* is at rest at the center of the Galaxy, we determine the circular rotation speed of the Galaxy at the position of the Sun to be 219 +/-- 20 km/s (for Ro = 8.0 kpc). Any peculiar proper motion of Sgr A* with respect to extragalactic sources is less than about 20 km/s. While the low luminosity of Sgr A*, for example, might possibly have come from a contact binary containing of order 10 solar masses, the lack of substantial peculiar motion rules out a "stellar" origin for Sgr A*. We estimate a lower limit to the mass of Sgr A* of about 1,000 solar masses. Even for this mass, Sgr A* appears to be radiating at less than 0.1 percent of its Eddington limit. Currently all observations are consistent with Sgr A* containing all of the nearly 2.6 million solar masses, deduced from stellar proper motions, in the form of a massive black hole.
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