Session 22 - Pierce Prize Lecture: Ghez.
Invited session, Monday, June 08
Presidio,

## [22.01] High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy

A. M. Ghez (UCLA)

More than a quarter century ago Lynden-Bell amp; Rees (1971), extrapolating from the idea that the highly energetic phenomena observed in very active galaxies are powered by massive central black holes, suggested 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. To probe the inner region of the Galaxy, it is crucial to attain the highest resolution possible. However, turbulence in the Earth's atmosphere distorts astronomical images and typically limits the angular resolution of long-exposures to \sim0.5 - 1 arcsec, an order of magnitude worse than the theoretical limit for large ground-based telescopes. In contrast to long exposures, short exposures, although distorted, preserve high spatial resolution information which can be used to recover diffraction limited images via a number of different techniques, such as the relatively simple and straight forward method of Shift-and-Add". Eckart amp; Genzel (1996, 1997) applied this method to data from the ESO 3-m NTT and achieved a resolution of 0\tt''15 in the first proper motion study of the central stellar cluster. This technique applied to data obtained from the W. M. Keck 10-meter telescope provides a unique opportunity to study the Galaxy center at an unprecedented resolution of 0\tt''05. I will report the initial results of a proper motion study of the Galaxy's central stellar cluster. Within our 6\tt'' \times 6\tt'' field of view, the motions of 90 stars are tracked over two years. With two-dimensional velocities as high as 1,400 km/sec, 0.5% the speed of light, these stars imply a central mass of 2.6 \pm 0.2 \times 10^6 M_ødot interior to a radius of \sim 0.01 pc, or densities in excess of 10^12 M_ødot / 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 supermassive 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.