Full 3-D Orbital Solutions for Stars Making a Close Approach
to the Supermassive Black Hole at the Center of the Galaxy
Professor Andrea Ghez
UCLA
4:15-5:15 Tuesday
February 18,
2003
Abstract
More than a quarter century ago, it was suggested that galaxies such
as our own Milky Way may harbor massive, though possibly dormant,
central black holes. Definitive proof, for or against, the existence
of a massive central black hole lies in the assessment of the
distribution of matter in the central few parsecs of the Galaxy. The
motion of the stars in the vicinity of the putative black hole offers
a robust method for accomplishing this task, with the objects located
closest to the Galactic Center providing the strongest constraints on
the black hole hypothesis.
Since it is the angular resolution that dictates the minimum radius at
which stars can be observed, it is crucial to attain the highest
resolution possible. Adaptive Optics techniques on the W. M. Keck
10-meter telescope, the largest optical/infrared telescope in the
world, provides a unique opportunity to study the Galaxy center at an
unprecedented resolution of 0.05 arcsec. Based on a high resolution
study that began in 1995, we have the most definitive evidence yet for
the existence of a supermassive black hole at the center of our
Galaxy. In particular, I will present two new and significant steps
forward in this near-infrared study of high velocity stars in the
central region of our Galaxy. Thanks to the advent of adaptive
optics, we have the first measurement of spectral absorption lines in
one of the high velocity stars, which just this year made near its
closest approach to the black hole. This provides new and fundamental
measure of the distance to the Galactic Center. With a 7-year
baseline of diffraction-limited imaging on the 10-m Keck telescopes
using both speckle imaging and adaptive optics, we have measured the
full 3-dimensional orbits for multiple stars, which are all making
close approaches to the supermassive black hole. The most dramatic
case is that of the newly identified star S0-16, which passes a mere
60 AU (or 0.0003 pc) from the center of the dark mass at a velocity of
9,000 km/sec. This provides the strongest case yet for the presence
of a supermassive black hole at the center of the Galaxy. With
multiple orbits, we are also now able to take an in-depth look at the
question of where these relatively young stars formed, which raises a
number of paradoxical problems.