Unambiguous Evidence for Monstrous Black Hole at Center of Our Galaxy Presented by UCLA Astronomer Andrea Ghez
Answering one of astronomy's most important questions, UCLA astronomer Andrea Ghez reported today at a conference in Tucson, Ariz., that a monstrous black hole resides at the center of our Milky Way galaxy, with a mass more than two million times that of our sun.
The question of what lies at the center of our galaxy, 24,000 light years away, has been the subject of a raging debate among astronomers for more than a quarter-century. Scientists have suspected that the galactic center contains either a single "supermassive" black hole or a cluster of millions of smaller stellar remnants. Black holes are collapsed stars so dense that nothing can escape their gravitational pull, not even light.
"Our galaxy is rather mild-mannered and quiet, and was one of the least likely galaxies to have a black hole at its center," said Ghez, an associate professor of physics and astronomy at UCLA, who spoke at The Central Parsecs: Galactic Center Workshop '98. "Yet a supermassive black hole at the center of our galaxy is precisely what we have found. The evidence for the black hole is very strong. One implication is that massive black holes may be found at the center of almost all galaxies."
The Milky Way is one of approximately 100 billion galaxies containing at least 100 billion stars each.
In her research, Ghez used the 10-meter Keck I Telescope -- the world's largest optical an infrared telescope -- atop Mauna Kea in Hawaii to study the movement of 200 stars that are close to the galactic center. Ghez studied these stars each year since 1995, using a technique she refined called "infrared speckle interferometry."
"Black holes cannot be seen directly, but their influence on nearby stars is very visible and provides a signature," said Ghez, 33. "We have found that signature in the rapid movement of the 20 or so stars that are most affected by its gravitational influence."
These 20 stars are orbiting ever closer to the black hole at a blinding speed of up to three million miles per hour -- about 10 times the speed at which stars typically move. The rapid speed at which the stars closest to the galactic center are moving reveals that the mass of the black hole -- 2.6 million solar masses -- must be concentrated in a single object, she said.
The star that was closest to the black hole in 1995 has since disappeared. Ghez has a number of possible theories to explain its disappearance, ranging from the mundane to the exotic. One explanation for observing a bright source in only one image, Ghez said, is that it was a "gravitational lensing event," which occurs when the light path from a star passing behind the black hole is bent by the strong gravitational field of the black hole. Alternatively, it could have been a flare due to a star falling into the black hole. Ghez, however, acknowledges that scientists may never learn which theory is correct.
One reason why astronomers previously had been unable to determine whether a black hole is at the galactic center is that our atmosphere distorts the images of stars. Ghez's speckle interferometry involves taking thousands of very quick, high-resolution snapshots that correct for the distortions produced by the Earth's atmosphere. She has developed algorithms -- specific computer commands based on sophisticated mathematics -- and software for analyzing the data.
Using traditional imaging techniques at the center of the galaxy would cause the stars closest to the galactic center to look fuzzy and indecipherable. Ghez's technique, however, improves the resolution by a factor of at least 20.
"The atmosphere blurs your vision," Ghez said, "but speckle interferometry clears the picture up; it's like putting on glasses. Think of seeing a coin that looks distorted at the bottom of a pond. We take thousands of freeze frames, and then can determine what is distorted and what is really at the bottom of the pond."
The center of the Milky Way was identified in 1968 by Eric Becklin, a UCLA professor of physics and astronomy. Its general location in the galaxy is known, but not its precise location. The center of the Milky Way 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.
Ghez studied the stars closest to the galactic center using the W.M. Keck Observatory's 10-meter Keck Telescope, and has returned to the Keck Observatory four times this year to observe the movement of these stars. She has been able to accurately predict the locations of the stars closest to the galactic center. (She identifies the stars based on their location and brightness.) Ghez has the highest resolution images of the galactic center ever obtained, which allow precise measurement of a group of stars close to the galactic center. Keck's large diameter allows Ghez to see fine details and to position the stars more accurately than a smaller telescope would permit -- details which were crucial in establishing the existence of the supermassive black hole.
"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, Ghez added, that it could detect two flies in Japan that are less than 10 feet away from each other. "That's the resolution we are reaching," she said, "if you scale it out to 24,000 light years."
Ghez's research is supported by the National Science Foundation through an NSF Young Investigator Award, the Packard Foundation and the Alfred P. Sloan Foundation.
"Ghez's research is a Areal tour-de-force," said Ferdinand Coroniti, chair of UCLA's physics and astronomy department. "She continues to dazzle and amaze the astronomical community with her technical virtuosity and scientific accomplishments."
She is now searching for additional black holes or other dark matter near the massive black hole. Her research has been accepted for publication in the December issue of Astrophysical Journal. Ghez's co-authors on the paper are former UCLA graduate student Beth Klein and UCLA astronomy professors Mark Morris and Eric Becklin.