GCNEWS
A Newsletter for Galactic Center Research
This Volume was edited by Angela Cotera & Heino Falcke
email: gcnews@aoc.nrao.edu
GCNEWS
The only object that can hardly be traced back to stellar activity in the GC is Sgr A* and for more than a decade now, we have strong evidence that there is a dark mass of ~106Mo in the center. For a long time the review paper by Genzel & Townes (1987, ARA&A) was often quoted in this respect since it showed an enclosed mass versus radius diagram that seemed to indicate a dark ``point'' mass of 2~106Mo in the GC. Since then this diagram has been continuously improved and continued to smaller radii - down to fractions of a parsec. Remarkably, regardless which method is used, the estimated mass never changed by much more than a factor of two, even though it was occasionally remarked that the data could in principle also be interpreted without any central object.
A few years ago the group at the MPE in Garching made headlines with their speckle images of the GC and the discovery of an alleged NIR counterpart to Sgr A* (ApJ, 407, 77). As it turned out later, those NIR sources were not Sgr A* itself but a cluster of stars very close to Sgr A*. Eckart & Genzel have now turned this into a big advantage: their paper (Nature 383, 415) crowns a four year effort to image the GC and to measure the proper motions of stars. Their most tantalizing result perhaps, is the possible detection of a high proper motion star (v~1600 km/sec) within the central cluster previously thought to be Sgr A*. The proper motions of the other, more distant stars together with infrared spectroscopy (Haller et al. 1996, Genzel et al. 1996) now give us the first reliable estimate for the gravitational potential well inside the central parsec.
In the recent Science News (150, 212), a weekly publication of noteworthy general science advances, the article by Eckart & Genzel and the work by Stolovy et al., included in the last GCNEWS, were reported under the headline ``Firm Evidence of Milky Way Black Hole''. In the four years since the MPE group began their work, many other Galaxies have made headlines because of claimed black hole detections with much less substantive evidence and it appears now that, mainly thanks to the PR efforts of the Space Telescope Science Institute, those claims (1, 2, 3) have saturated the popular market. This may explain, why the paper by Eckart & Genzel did not attract the popular media attention it merits. Their paper certainly sets a milestone in the long quest to determine the central mass.
While the fact that the authors specify the mass in the GC with three significant digits may still be amusing to some, it makes clear that the discussion has now reached a stage where we are no longer discussing orders of magnitude, but rather second and third significant digits. Moreover, their work has removed some of the ambiguity spectroscopy alone is unable to resolve. The GC is now the best supermassive black hole candidate - besides NGC 4258 - we know of, and the high proper motion of the stars can make this even better in the future. As reported in Vol. 1 of GCNEWS, HST/NICMOS will dedicate a lot of time to the same question, and the Keck telescope will undoubtablly add to this endeavour (see Klein et al. GCNEWS Vol. 2.).
Hence, the GC will remain a hot topic in the coming years and one can hope that in the wake of these discoveries other important parts of the GC research, despite less public attention, will prosper equally well. Thanks to the many colleagues who are contributing their abstracts, GCNEWS will be here to keep you up on the latest in this exciting field.