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

Abstract

Over a three year period (1995-8), we have conducted a diffraction-limited imaging study at 2.2 microns of the inner 6tt'' times 6tt'' of the Galaxy's central stellar cluster using the W. M. Keck 10-m telescope. Stars moving with velocities as large as 1,400 +/- 100 km/sec have been detected. 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*, suggesting that Sgr A* is coincident (+/- 0 farcs 1) with the dynamical center of the Galaxy. As a function of distance from Sgr A*, the velocity dispersion displays a falloff well fit by Keplerian motion (σv ~r{-0.5 +/- 0.1}) about a central dark mass of 2.6 (+/- 0.2) times 106 Modot confined to a volume of at most 10-6 pc3, consistent with earlier results. Thus, independent of the presence of Sgr A*, the large inferred central density of at least 1012 Modot/pc3, 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.