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                              G C N E W S
                             * Newsflash *

            - The Newsletter for Galactic Center Research -

gcnews@mpifr-bonn.mpg.de             http://www.mpifr-bonn.mpg.de/gcnews
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Vol. 8, No. 15                                              Jul 21, 1998


Recently submitted papers:
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Email    : ghez@athena.astro.ucla.edu
Title    : High Proper Motion Stars in the Vicinity of Sgr A*:  
           Evidence for a Supermassive Black Hole at the Center of 
           Our Galaxy
Author(s): A. M. Ghez(2,3), B. L. Klein(1), M. Morris(1), \& E.E. Becklin(1)
Institute: (1) Dept. of Physics and Astronomy, UCLA, Los Angeles, CA 
           90095-1562   (2) Sloan Fellow   (3) Packard Fellow
Paper    : ApJ, accepted
EPrint   : astro-ph/9807210

Abstract:
Over a two year period, we have conducted a diffraction-limited
imaging study at 2.2 mu m of the inner 6'' * 6'' of the Galaxy's
central stellar cluster using the W. M. Keck 10-m telescope.  The K
band images obtained in 1995 June, 1996 June, and 1997 May have the
highest angular resolution obtained at near-infrared wavelengths from
ground or space ( theta _res = 0.'' 05 = 0.002 pc) and reveal a large
population of faint stars.  We use an unbiased approach for
identifying and selecting stars to be included in this proper motion
study, which results in a sample of 90 stars with brightness ranging
from K = 9 to 17 mag and two-dimensional velocities as large as 1,400
+/- 100 km/sec.  Compared to earlier work (Eckart et al. 1997; Genzel
et al. 1997), the source confusion is reduced by a factor of 9, the
number of stars with proper motion measurement in the central 25
arcsec^2 of our galaxy is doubled, and the accuracy of the velocity
measurements in the central 1 arcsec^2 is improved by a factor of 4.
The peaks of both the stellar surface density and the velocity
dispersion are consistent with the position of the unusual radio
source and black hole candidate, Sgr A*, suggesting that Sgr A* is
coincident (+/- 0.'' 1) with the dynamical center of the Galaxy.  As a
function of distance from Sgr A*, the velocity dispersion displays a
falloff well fit by Keplerian motion ( sigma _v ~ r^-0.5 +/- 0.1)
about a central dark mass of 2.6 (+/- 0.2) * 10^6 M_o confined to a
volume of at most 10^-6 pc^3, consistent with earlier results.
Although uncertainties in the measurements mathematically allow for
the matter to be distributed over this volume as a cluster, no
realistic cluster is physically tenable.  Thus, independent of the
presence of Sgr A*, the large inferred central density of at least
10^12 M_o / pc^3, which exceeds the volume-averaged mass densities
found at the center of any other galaxy, leads us to the conclusion
that our Galaxy harbors a massive central black hole.
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			       Edited by
Angela Cotera                                Heino Falcke
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