Title: Observing the Fastest Moving Stars in Our Galaxy: Evidence for a Supermassive Central Black Hole
Speaker: Andrea Ghez
Abstract:
More than a quarter century ago Lynden-Bell & Rees (1971),
extrapolating from the idea that the highly energetic phenomena observed in
very active galaxies are powered by massive central black holes,
suggested ago that much less active
galaxies such as our own Milky Way may also harbor massive, though possibly
dormant, central black holes. Early on, indirect support for a central black
hole arose from the discovery of the unusual radio source Sgr A*; its
non-thermal spectrum, compact size, and lack of detected motion
led researchers to associate it with the putative black hole.
Definitive proof for the existence of a massive central black
hole and its association with Sgr A*, however, lies in the assessment
of the distribution of mass in the central few parsecs of the Galaxy.
Assuming that gravity is the dominant force, the motion of the stars
in the vicinity of the putative black hole offers a robust
method for accomplishing this task, by revealing
the mass interior to the orbital radius of the objects studied.
Thus, objects located closest to the Galactic Center
provide the strongest constraints on the black hole hypothesis.
I will report the initial results of a proper motion
study of the Galaxy's central stellar cluster based on
diffraction-limited images obtained with the Keck 10-meter telescope
(1995-present). The stars tracked provide strong evidence for
a central mass of $2.6 \pm 0.2 \times 10^6 M_{\odot}$
interior to a radius of $\sim$ 0.01 pc, or densities in excess of
$10^{12} M_{\odot} / pc^3$, exceeding the volume-averaged mass densities
inferred so far for the center of any other galaxy. The high mass to light
ratio and density leads us to conclude that our Galaxy harbors a massive
central black hole.
Our Galaxy was neither the first nor an obvious candidate for a central
supermassive black hole; however it, along with NGC 4258, has become one of
the strongest cases for a $10^6 M_{\odot}$ black hole.
The significance of a central black hole in our Galaxy is the implication
that massive black holes might be found at the centers of almost all galaxies.
Reference for students: Ghez et al. 1998 ApJ 509, 678 Lunch on Friday 4/23 at 12:30 in the classroom (A-101).