# April 22, 1999 CfA Colloquium

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).