Unveiling a Supermassive Black Hole at the Center of Our Galaxy Andrea Ghez, University of California, Los Angeles

The proximity of our Galaxy's center presents an opportunity to build a case for a supermassive black hole and to study the black hole's environment and its effects thereon with much higher spatial resolution than can be brought to bear on any other galaxy. After almost a decade of astrometry from diffraction-limited speckle imaging at the W. M. Keck 10 m telescope, we have moved the case for a supermassive black hole at the Galactic Center from a possibility to a certainty; this is based on our recent ability to determine the orbits of individual stars, which confines the central dark mass of 4 million times the mass of the sun to within 90 AU (1 AU = the Earth-Sun distance), or equivalently, 1,000 Schwarzchild radii. With the advent of adaptive optics, we have significantly expanded our studies of the Galaxy's central black hole through the addition of diffraction-limited spectroscopy and deep imaging at wavelengths other than 2.2 microns. Spectroscopy has revealed that the stars oribiting in such close proximity are apparently massive and young; the origin of these stars is difficult to explain, given the strong tidal forces, and may provide key insight into the growth of the central black hole. Thermal infrared imaging (3.8 microns) has led to the direct detection of plasma associated with the central black hole. This source is variable on timescales as short as 40 min, implying that the emission arises quite close to the black hole, within 5 AU, or 80 Schwarzchild radii. This provides a new, constantly accessible, window into the physical conditions of the plasma in close proximity to the central black hole.