Answering one of astronomy's most important questions, UCLA astronomerAndrea Ghez reported today at a conference in Tucson, Ariz., that a monstrousblack hole resides at the center of our Milky Way galaxy, with a mass morethan two million times that of our sun.
The question of what lies at the center of our galaxy, 24,000 lightyears away, has been the subject of a raging debate among astronomers formore than a quarter-century. Scientists have suspected that the galacticcenter contains either a single "supermassive" black hole ora cluster of millions of smaller stellar remnants. Black holes are collapsedstars so dense that nothing can escape their gravitational pull, not evenlight.
"Our galaxy is rather mild-mannered and quiet, and was one of theleast likely galaxies to have a black hole at its center," said Ghez,an associate professor of physics and astronomy at UCLA, who spoke at TheCentral Parsecs: Galactic Center Workshop '98. "Yet a supermassiveblack 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 thatmassive black holes may be found at the center of almost all galaxies."
The Milky Way is one of approximately 100 billion galaxies containingat least 100 billion stars each.
In her research, Ghez used the 10-meter Keck I Telescope -- the world'slargest optical an infrared telescope -- atop Mauna Kea in Hawaii to studythe movement of 200 stars that are close to the galactic center. Ghez studiedthese stars each year since 1995, using a technique she refined called"infrared speckle interferometry."
"Black holes cannot be seen directly, but their influence on nearbystars is very visible and provides a signature," said Ghez, 33. "Wehave found that signature in the rapid movement of the 20 or so stars thatare most affected by its gravitational influence."
These 20 stars are orbiting ever closer to the black hole at a blindingspeed of up to three million miles per hour -- about 10 times the speedat which stars typically move. The rapid speed at which the stars closestto 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, rangingfrom the mundane to the exotic. One explanation for observing a brightsource in only one image, Ghez said, is that it was a "gravitationallensing event," which occurs when the light path from a star passingbehind the black hole is bent by the strong gravitational field of theblack hole. Alternatively, it could have been a flare due to a star fallinginto the black hole. Ghez, however, acknowledges that scientists may neverlearn which theory is correct.
One reason why astronomers previously had been unable to determine whethera black hole is at the galactic center is that our atmosphere distortsthe images of stars. Ghez's speckle interferometry involves taking thousandsof very quick, high-resolution snapshots that correct for the distortionsproduced by the Earth's atmosphere. She has developed algorithms -- specificcomputer commands based on sophisticated mathematics -- and software foranalyzing the data.
Using traditional imaging techniques at the center of the galaxy wouldcause the stars closest to the galactic center to look fuzzy and indecipherable.Ghez's technique, however, improves the resolution by a factor of at least20.
"The atmosphere blurs your vision," Ghez said, "but speckleinterferometry clears the picture up; it's like putting on glasses. Thinkof seeing a coin that looks distorted at the bottom of a pond. We takethousands of freeze frames, and then can determine what is distorted andwhat 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 thegalaxy is known, but not its precise location. The center of the MilkyWay is located due south in the summer sky.
The black hole at the center of our galaxy came into existence billionsof years ago, perhaps as very massive stars collapsed at the end of theirlife 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 KeckObservatory four times this year to observe the movement of these stars.She has been able to accurately predict the locations of the stars closestto the galactic center. (She identifies the stars based on their locationand brightness.) Ghez has the highest resolution images of the galacticcenter ever obtained, which allow precise measurement of a group of starsclose to the galactic center. Keck's large diameter allows Ghez to seefine details and to position the stars more accurately than a smaller telescopewould permit -- details which were crucial in establishing the existenceof the supermassive black hole.
"The Keck Observatory is the best facility in the world for thisresearch," Ghez said. "The Keck Telescope enables us to trackstars very precisely." The telescope's resolution is so high, Ghezadded, that it could detect two flies in Japan that are less than 10 feetaway 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 throughan NSF Young Investigator Award, the Packard Foundation and the AlfredP. Sloan Foundation.
"Ghez's research is a Areal tour-de-force," said FerdinandCoroniti, chair of UCLA's physics and astronomy department. "She continuesto dazzle and amaze the astronomical community with her technical virtuosityand scientific accomplishments."
She is now searching for additional black holes or other dark matternear the massive black hole. Her research has been accepted for publicationin the December issue of Astrophysical Journal. Ghez's co-authors on thepaper are former UCLA graduate student Beth Klein and UCLA astronomy professorsMark Morris and Eric Becklin.