Research

The proximity of our Galaxy's center (only 8 kpc) presents a unique opportunity to study the environment of a supermassive black hole with much higher spatial resolution than can be brought to bear on any other galaxy. In 1995, we initiated a diffraction-limited study on the W. M. Keck 10-meter telescope, of the Galaxy's central cluster. During this program, we have measured the motions of stars on the plane of the sky and have dramatically improved the case for a supermassive black hole at the Galactic Center, which has evolved through 3 distinct stages of confidence:

1. beginning as a possibility, when the earlier, low angular resolution, dynamical measurements of the gas and stars at the center of the Milky Way suggested the presence of 10 million solar masses (10^6) of dark matter and confined it to within a radius of ~0.1 pc
2. becoming a strong probability, when proper motion measurements increased the inferred dark mass density by 3 orders of magnitude to 10^12 M_sun/pc^3, thereby eliminating a cluster of dark objects as a possible explanation of the Galaxy's central dark mass concentration, and finally
3. crystalizing to a certainty when individual stellar orbits confined the central dark mass to within 0.0004 pc (90 AU), thus increasing the dark mass density by another four orders of magnitude, and eliminating the fermion ball hypothesis as an alternative to a single supermassive black hole.

The unusual radio source SgrA* -- the first known observational manifestation of the black hole -- coincides precisely with the location we infer for the dark mass concentration, as does the X-ray counterpart recently found with Chandra. Many attempts have thus been made to understand the overall spectrum of SgrA* in terms of accretion and/or outflow models, but it has been difficult to apply critical tests of these models in the absence of a detection at near-, mid- or far-infrared wavelengths. This situation has recently evolved quite dramatically. The advent of Adaptive Optics (AO) and a new facility class instrument (NIRC2) at Keck in 2002 led to a number of exciting new results, including the first infrared detection of Sgr A*. This, plus the short-term variability of the emission at several of the observed wavelengths brings us to a promising new era when observations can strongly constrain the wide array of extant models.

The new instrumentation has also yielded the first detection of a spectral line from one of the stars in close proximity to the black hole, and hence a firm spectral classification as an early-type star (Ghez et al. 2003a). This finding not only adds the dimension of radial velocity to our orbital determinations, but it also severely aggravates the already troubling question of how young stars can be present within the gravitational domain of influence of the black hole.

One of the most intriguing puzzles to stem from a definitive case for a supermassive black hole at the center of the Milky Way is the origin of the apparently young stars in the central parsec; the existing gas in this region is far from being sufficiently dense for self-gravity to overcome the strong tidal forces from the black hole, making star formation an unlikely process unless past densities in the central parsec were far more extreme than at present. A wide range of possible solutions have been proposed to account for the apparently young stars in the vicinity of a supermassive black hole. One possibility is that the local gas density was much higher in the past, allowing gravitational collapse to occur in the presence of a strong tidal field. This could in principle occur through a pre-existing accretion disk, which has since dispersed or by the collision of infalling dense gas clouds although no such sufficiently dense gas clouds have been observed. Alternatively, stars might form at larger radii, where the tidal forces are much lower, and migrate inwards through dynamical friction. For this to work, the stars would have to belong to a stellar cluster so dense that it would have undergone core collapse, possibly forming an intermediate mass black hole, which would act as In a dramatically different approach, it has also been suggested that these stars are not truly young but are old stars that have been altered by their environment. At present, none of the proposed theories is altogether satisfactory, leaving both the He~I emission-line stars and the Sgr A* cluster stars as paradoxes of apparent youth in the vicinity of a supermassive black hole.

Our current research focuses on a number of new questions which have arisen in the course of our recent research regarding the properties and environs of the Galaxy's central supermassive black hole:

• What is the distance to the Galactic center, R_o?
• How did the apparently young stars in the black hole's vicinity form?
• Is there an extended distribution of dark matter surrounding the central black hole?
• What physical processes give rise to the variable emission from the black hole at near-IR, millimeter and X-ray wavelengths, and why is the accretion flow so underluminous?