nai

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, thanks to 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 orbiting 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 and providing a new, constantly accessible, window into
the physical conditions of the plasma in close proximity to the central
black hole.

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