| Harvard-Smithsonian Center for Astrophysics: Publications and Public Affairs |
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Release No.: 98-01
Embargoed until:
WASHINGTON, DC -- High-resolution observations with the Very Long Baseline Array (VLBA) radio telescope show that the unusual radio source called Sagittarius A* (Sgr A*) seems to be a massive black hole anchored at the very center of our Milky Way galaxy. These results, in conjunction with other work, were presented at the American Astronomical Society meeting here today by Mark Reid of the Harvard-Smithsonian Center for Astrophysics on behalf of an international team of astronomers including Anthony Readhead of Caltech, Rene Vermeulen from the Netherlands, and Robert Treuhaft of the Jet Propulsion Laboratory. Because of its similarity to the active nuclei of other galaxies, astronomers have long suspected that Sgr A*, an extremely bright, point-like source of radio emission could be a massive black hole. However, the total power emitted by Sgr A* is comparatively low, less than emitted by some rare interacting stars. Thus, based on the strength of its emissions, Sgr A* does not have to be a very massive object. Recently, the motions of stars very close to Sgr A* were measured by a group led by Andreas Eckart and Reinhard Genzel of Germany's Max-Planck-Institut fur Extraterrestrische Physik. They found extremely fast motions, some exceeding 1000 km/second, which would require a total mass nearly three million times that of the Sun centered at the position of Sgr A* and within a region of space only about 100 times larger than our Solar System. What is the nature of this extraordinary mass concentration? Is Sgr A* a gigantic black hole, or simply an unusual group of stars? One way to determine this is to measure the motion of Sgr A* itself. If it is a massive black hole, it should stay anchored to the center of the Milky Way. On the other hand, if it is a single star (or small group of stars), then, like other stars in its vicinity, Sgr A* should be moving very rapidly. The results presented at the AAS meeting include images of Sgr A* with the Very Long Baseline Array (VLBA), the National Science Foundation-supported array of radio telescopes which spans the USA from Hawaii to New England. These observations provided enough resolution to see Sgr A* move by many diameters in one year. After tracking Sgr A* for two years, Reid and his collaborators found that most of its apparent motion could be attributed to the Sun's orbit about the center of the Milky Way. (Although it takes over 200 million years for the Sun to completely circle the Milky Way, the effects of its orbit can be detected in only 10 days by VLBA observations!) After correcting for solar effects, the remaining motion of Sgr A* is less than 20 km/sec, even slower than the Earth orbits the Sun. This result confirms similar measurements made with less intrinsic accuracy, but over a longer time period, using the Very Large Array by Don Backer of UC Berkeley and Dick Sramek of the National Radio Astronomy Observatory. Such a low speed rules out the option that Sgr A* is any single star, or even a small group of stars. From the upper limit on the motion of Sgr A*, the astronomers conclude that its mass is certainly larger than a few thousand, and more likely some three million, times that of the Sun. The results are totally consistent with the theory that Sgr A* is a massive black hole anchoring the center of the Milky Way. For more information, contact:
Mark Reid, CfA, mreid@cfa.harvard.edu, 617-495-7470 |