High Proper-Motion Stars in the Vicinity of Sagittarius A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy

Over a 2 year period we have conducted a di†raction-limited imaging study at 2.2 km of the inner 6A ) 6A of the central stellar cluster of the Galaxy using the W. M. Keck 10 m telescope. The K-band images obtained in 1995 June, 1996 June, and 1997 May have the highest angular resolution obtained at near-infrared wavelengths from ground or space pc) and reveal a large population of (h res \ 0A\0.002 faint stars. We use an unbiased approach for identifying and selecting stars to be included in this proper- motion study, which results in a sample of 90 stars with brightness ranging from K \ 9E17 mag and two-dimensional velocities as large as 1400 ^ 100 km s~1. Compared to earlier work et al. (Eckart 1997; et al. the source confusion is reduced by a factor of 9, the number of stars with proper- Genzel 1997), motion measurement in the central 25 arcsec2 of our Galaxy is doubled, and the accuracy of the velocity measurements in the central 1 arcsec2 is improved by a factor of 4. The peaks of both the stellar surface density and the velocity dispersion are consistent with the position of the unusual radio source and black hole candidate Sgr A*, which suggests that Sgr A* is coincident with the dynamical center (^0A.1) of the Galaxy. As a function of distance from Sgr A*, the velocity dispersion displays a fallo† well--tted by Keplerian motion about a central dark mass of 2.6 ^ 0.2 ) 106 con-ned to a (p v D r~0.5B0.1) M _ volume of at most 10~6 pc3, which is consistent with earlier results. Although uncertainties in the mea- surements mathematically allow for the matter to be distributed over this volume as a cluster, no realis- tic cluster is physically tenable. Thus, independent of the presence of Sgr A*, the large inferred central density of at least 1012 pc~3, which exceeds the volume-averaged mass densities found at the center M _ of any other galaxy, leads us to the conclusion that our Galaxy harbors a massive central black hole. Subject headings: black hole physics E Galaxy: center E Galaxy: kinematics and dynamics E infrared: stars E stars: kinematics E techniques: image processing

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