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 .05 \ 0.002 faint stars. We use an unbiased approach for identifying and selecting stars to be included in this propermotion study, which results in a sample of 90 stars with brightness ranging from K \ 917 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 conned 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 measurements mathematically allow for the matter to be distributed over this volume as a cluster, no realistic 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.