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 diffraction-limited imaging study at 2.2 μm of the inner 6'' × 6'' 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 (θres = 0.″05 = 0.002 pc) and reveal a large population of 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 = 9-17 mag and two-dimensional velocities as large as 1400 ± 100 km s-1. Compared to earlier work (Eckart et al. 1997; Genzel et al. 1997), the source confusion is reduced by a factor of 9, the number of stars with proper-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 (±0.″1) with the dynamical center of the Galaxy. As a function of distance from Sgr A*, the velocity dispersion displays a falloff well-fitted by Keplerian motion (σv ~ r-0.5±0.1) about a central dark mass of 2.6 ± 0.2 × 106 M☉ confined to a 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 M☉ pc-3, which exceeds the volume-averaged mass densities found at the center of any other galaxy, leads us to the conclusion that our Galaxy harbors a massive central black hole.