UCLA astronomer Andrea Ghez announced more than four years ago that a monstrous black hole resides at the center of our Milky Way galaxy, 24,000 light years away, with a mass more than 2 million times that of our sun. Some astronomers greeted the announcement with skepticism, and proposed exotic forms of matter as alternatives.
At the American Association for the Advancement of Science meeting Feb. 16 in Denver, Ghez reported that the case for the black hole has been strengthened substantially, and that all of the proposed alternatives can be excluded.
"The case for the supermassive black hole was strong before, and we have substantially improved it," said Ghez, professor of physics and astronomy at UCLA. "Now it's a 99.99 percent certainty. We can rule out every alternative that has been proposed."
The case is made most strongly on the basis of a newly discovered star, S0-16, which passed within 60 AU (just slightly larger than the distance between the sun and Pluto) of the black hole, moving at approximately 52 million miles per hour -- the highest velocity observed yet at the galactic center.
Since 1995 Ghez has been using the W.M. Keck Observatory's 10-meter Keck I Telescope atop Mauna Kea in Hawaii -- the world's largest optical and infrared telescope -- to study the movement of 200 stars close to the galactic center. She has made measurements using a technique she refined called infrared speckle interferometry, and for the last few years, an even more sophisticated technique, called adaptive optics, which enables her to see more of the densely packed stars in this region.
One surprising result Ghez learned from the spectroscopy is that the stars closest to the black hole appear to very young -- less than 10 million years old. In contrast, our galaxy is about 10 to the 10th power years old.
"The very existence of some of these stars is a paradox, especially eight young stars close to the black hole," Ghez said. "The tidal force the black hole exerts on the stars makes it difficult to understand how star formation occurred in that environment. These stars have been incredibly important in revealing the presence of the black hole; now we want to understand the mystery of how these stars formed and why they look so young. In the current configuration, there is no way star formation should be able to occur.
"One possible explanation may be that the stars are not really that young, and that their proximity to the black hole has altered their appearance; that is, they may be old stars masquerading as young stars, stars that have experienced astronomical botox.
"However, it is difficult to invoke any mechanism where the black hole could have that much influence on the surrounding stars. The alternative is that the stars really are young, but how you get stars to form that close to the black hole is very difficult. One idea is that they formed farther out and migrated inwards by interactions with other stars; that their orbits were altered. Because they are so young, however, they didn't have much time for that to happen.
"We are looking look for deviations from Keplerian orbits -- the expected orbit for a star going around a black hole -- to search for the presence of an entourage of smaller stellar black holes or neutron stars surrounding the supermassive black hole; deviations would suggest a population of dark stars that could help these stars migrate inwards. The region may be much more crowded than we think."
Ghez and her colleagues have detected the orbits for eight stars close to the galactic center. From the orbits, they are able to calculate the mass for the supermassive black hole. They have found a substantially greater density of mysterious dark matter in the region than they knew of in 2000, when they published in Nature that three stars have accelerated by more than 250 thousand miles per hour per year as they orbit the black hole; that was the first time astronomers ever saw stars accelerate around a supermassive black hole. They learned the location of the black hole from the acceleration.
Ghez is now searching for additional black holes or other dark matter near the massive black hole.
One of the stars will complete its orbit around the supermassive black hole in just 15 years.
"The light from these stars takes 24,000 light years to get to us," she said, "and we're talking about a complete orbit in 15 years." (Ghez and her colleagues reported in 2000 that they believed the orbit was 15 years, and that figure has since been confirmed.)
Black holes are collapsed stars so dense that nothing can escape their gravitational pull, not even light. Black holes cannot be seen directly, but their influence on nearby stars is visible, and provides a signature, Ghez said. The black hole, with a mass 2.6 million times that of our sun, is in the constellation of Sagittarius.
"We have found that signature in the rapid movement of the stars that are most affected by its gravitational influence," she said. Twenty stars near the galactic center are orbiting ever closer to the black hole at a blinding speed of up to 3 million miles per hour -- about 10 times the speed at which stars typically move.
"We know the location of the black hole so precisely," Ghez said, "that it's like someone in Los Angeles who can identify where someone in Boston is standing to within the width of her hand, if you scale it out to 24,000 light years."
The Milky Way is one of approximately 100 billion galaxies containing at least 100 billion stars each.
Ghez's colleagues include UCLA physics and astronomy professors Mark Morris and Eric Becklin. Becklin identified the center of the Milky Way in 1968. The galactic center is located due south in the summer sky.
The black hole at the center of our galaxy came into existence billions of years ago, perhaps as very massive stars collapsed at the end of their life cycles and coalesced into a single, supermassive object.
"The Keck Observatory is the best facility in the world for this research," Ghez said. "The Keck Telescope enables us to track stars very precisely." The telescope's resolution is so high, she said, that it could detect two flies in Japan that are less than 10 feet away from each other.
Ghez's research is supported by the National Science Foundation and the Packard Foundation.
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