David Aguilar
(617) 495-7462

Christine Pulliam
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CfA Press Release
 
 Release No.: 98-01

Embargoed until:
9:30 am, January 7, 1998

THE BLACKHOLE AT THE CENTER OF THE MILKY WAY

WASHINGTON, DC -- High-resolution observations with the Very LongWASHINGTON, DC -- High-resolution observations with the Very LongBaseline Array (VLBA) radio telescope show that the unusual radioBaseline Array (VLBA) radio telescope show that the unusual radiosource called Sagittarius A* (Sgr A*) seems to be a massive blacksource called Sagittarius A* (Sgr A*) seems to be a massive blackhole anchored at the very center of our Milky Way galaxy. hole anchored at the very center of our Milky Way galaxy.

These results, in conjunction with other work, were presented at theThese results, in conjunction with other work, were presented at theAmerican Astronomical Society meeting here today by Mark Reid of theAmerican Astronomical Society meeting here today by Mark Reid of theHarvard-Smithsonian Center for Astrophysics on behalf of anHarvard-Smithsonian Center for Astrophysics on behalf of aninternational team of astronomers including Anthony Readhead ofinternational team of astronomers including Anthony Readhead ofCaltech, Rene Vermeulen from the Netherlands, and Robert Treuhaft ofCaltech, Rene Vermeulen from the Netherlands, and Robert Treuhaft ofthe Jet Propulsion Laboratory.the Jet Propulsion Laboratory.

Because of its similarity to the active nuclei of other galaxies,Because of its similarity to the active nuclei of other galaxies,astronomers have long suspected that Sgr A*, an extremely bright,astronomers have long suspected that Sgr A*, an extremely bright,point-like source of radio emission could be a massive black hole. point-like source of radio emission could be a massive black hole. However, the total power emitted by Sgr A* is comparatively low, However, the total power emitted by Sgr A* is comparatively low, less than emitted by some rare interacting stars. Thus, basedless than emitted by some rare interacting stars. Thus, basedon the strength of its emissions, Sgr A* does not have to be a veryon the strength of its emissions, Sgr A* does not have to be a verymassive object.massive object.

Recently, the motions of stars very close to Sgr A* were measured 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 by a group led by Andreas Eckart and Reinhard Genzel of Germany's Max-Planck-Institut fur Extraterrestrische Physik. They found Max-Planck-Institut fur Extraterrestrische Physik. They found extremely fast motions, some exceeding 1000 km/second, which would extremely fast motions, some exceeding 1000 km/second, which would require a total mass nearly three million times that of the Sun 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 centered at the position of Sgr A* and within a region of space only about 100 times larger than our Solar System. about 100 times larger than our Solar System.

What is the nature of this extraordinary mass concentration? Is Sgr What is the nature of this extraordinary mass concentration? Is Sgr A* a gigantic black hole, or simply an unusual group of stars? One 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 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 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 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* group of stars), then, like other stars in its vicinity, Sgr A* should be moving very rapidly.should be moving very rapidly.

The results presented at the AAS meeting include images of Sgr A*The results presented at the AAS meeting include images of Sgr A*with the Very Long Baseline Array (VLBA), the National Sciencewith the Very Long Baseline Array (VLBA), the National ScienceFoundation-supported array of radio telescopes which spans the USAFoundation-supported array of radio telescopes which spans the USAfrom Hawaii to New England. These observations provided enoughfrom Hawaii to New England. These observations provided enoughresolution to see Sgr A* move by many diameters in one year. Afterresolution to see Sgr A* move by many diameters in one year. Aftertracking Sgr A* for two years, Reid and his collaborators found thattracking Sgr A* for two years, Reid and his collaborators found thatmost of its apparent motion could be attributed to the Sun's orbit 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 about the center of the Milky Way. (Although it takes over 200 million years for the Sun to completely circle the Milky Way, million years for the Sun to completely circle the Milky Way, the effects of its orbit can be detected in only 10 days by VLBAthe effects of its orbit can be detected in only 10 days by VLBAobservations!)observations!)

After correcting for solar effects, the remaining motion of Sgr After correcting for solar effects, the remaining motion of Sgr A* is less than 20 km/sec, even slower than the Earth orbits A* is less than 20 km/sec, even slower than the Earth orbits the Sun. This result confirms similar measurements made with the Sun. This result confirms similar measurements made with less intrinsic accuracy, but over a longer time period, using the less intrinsic accuracy, but over a longer time period, using the Very Large Array by Don Backer of UC Berkeley and Dick Sramek of Very Large Array by Don Backer of UC Berkeley and Dick Sramek of the National Radio Astronomy Observatory. Such a low speed rules the National Radio Astronomy Observatory. Such a low speed rules out the option that Sgr A* is any single star, or even a small out the option that Sgr A* is any single star, or even a small group of stars. group of stars.

From the upper limit on the motion of Sgr A*, the astronomers From the upper limit on the motion of Sgr A*, the astronomers conclude that its mass is certainly larger than a few thousand, and conclude that its mass is certainly larger than a few thousand, and more likely some three million, times that of the Sun. The results 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 are totally consistent with the theory that Sgr A* is a massive black hole anchoring the center of the Milky Way. hole anchoring the center of the Milky Way.

For more information, contact:

Mark Reid, CfA, mreid@cfa.harvard.edu, 617-495-7470
James Cornell, CfA, jcornell@cfa.harvard.edu, 617-495-7462


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