Science + Technology

Case for Massive Black Hole at Center of Milky Way Strengthened, Alternatives Excluded, UCLA Astronomer Andrea Ghez Reported at AAAS


UCLA astronomer Andrea Ghez announced more than four years agothat 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 exoticforms of matter as alternatives.

At the American Associationfor the Advancement of Science meeting Feb. 16 in Denver, Ghez reported thatthe case for the black hole has been strengthened substantially, and that allof the proposed alternatives can be excluded.

"The case for the supermassive black hole was strong before, andwe have substantially improved it," said Ghez, professor of physics andastronomy at UCLA. "Now it's a 99.99 percent certainty. We can rule out everyalternative that has been proposed."

The case is made most strongly on the basis of a newly discoveredstar, S0-16, which passed within 60 AU (just slightly larger than the distancebetween the sun and Pluto) of the black hole, moving at approximately 52million miles per hour — the highest velocity observed yet at the galacticcenter.

Since 1995 Ghez has been using the W.M. Keck Observatory's10-meter Keck I Telescope atop Mauna Kea in Hawaii — the world's largestoptical and infrared telescope — to study the movement of 200 stars close tothe galactic center. She has made measurements using a technique she refinedcalled infrared speckle interferometry, and for the last few years, an evenmore sophisticated technique, called adaptive optics, which enables her to seemore of the densely packed stars in this region.

One surprising result Ghez learned from the spectroscopy is thatthe stars closest to the black hole appear to very young — less than 10 millionyears 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 tidalforce the black hole exerts on the stars makes it difficult to understand howstar formation occurred in that environment. These stars have been incrediblyimportant in revealing the presence of the black hole; now we want tounderstand the mystery of how these stars formed and why they look so young. Inthe current configuration, there is no way star formation should be able tooccur.

"One possible explanation may be that the stars are not reallythat young, and that their proximity to the black hole has altered theirappearance; that is, they may be old stars masquerading as young stars, starsthat have experienced astronomical botox.

"However, it is difficult to invoke any mechanism where the blackhole could have that much influence on the surrounding stars. The alternativeis that the stars really are young, but how you get stars to form that close tothe black hole is very difficult. One idea is that they formed farther out andmigrated inwards by interactions with other stars; that their orbits werealtered. Because they are so young, however, they didn't have much time forthat to happen.

"We are looking look for deviations from Keplerian orbits — theexpected orbit for a star going around a black hole — to search for thepresence of an entourage of smaller stellar black holes or neutron starssurrounding the supermassive black hole; deviations would suggest a populationof dark stars that could help these stars migrate inwards. The region may bemuch more crowded than we think."

Ghez and her colleagues have detected the orbits for eight starsclose to the galactic center. From the orbits, they are able to calculate themass for the supermassive black hole. They have found a substantially greater density of mysterious dark matter in theregion than they knew of in 2000, when they published in Nature that threestars have accelerated by more than 250thousand miles per hour per year as they orbit the black hole; that wasthe first time astronomers ever saw stars accelerate around a supermassiveblack hole. They learned the location of the black hole from the acceleration.

Ghez is now searching for additional black holes or other darkmatter near the massive black hole.

One of the stars will complete its orbit around the supermassiveblack hole in just 15 years.

"The light from these stars takes 24,000 light years to get tous," she said, "and we're talking about a complete orbit in 15 years." (Ghezand 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 theirgravitational pull, not even light. Black holes cannot be seen directly, buttheir influence on nearby stars is visible, and provides a signature, Ghezsaid. The black hole, with a mass 2.6 million times that of our sun, is in theconstellation of Sagittarius.

"We have found that signature in the rapid movement of the starsthat are most affected by its gravitational influence," she said. Twenty starsnear the galactic center are orbiting ever closer to the black hole at ablinding speed of up to 3 million miles per hour — about 10 times the speed atwhich stars typically move.

"We know the location of the black hole so precisely," Ghez said,"that it's like someone in Los Angeles whocan identify where someone in Boston is standing to within the width of herhand, if you scale it out to 24,000 light years."

The Milky Way is one of approximately 100 billion galaxiescontaining at least 100 billion stars each.

Ghez's colleagues include UCLAphysics and astronomy professors Mark Morris and Eric Becklin. Becklinidentified the center of the Milky Way in 1968. The galactic center is locateddue south in the summer sky.

The black hole at the center of ourgalaxy came into existence billions of years ago, perhaps as very massive starscollapsed at the end of their life cycles and coalesced into a single,supermassive object.

"The Keck Observatory is the bestfacility in the world for this research," Ghez said. "The Keck Telescopeenables 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 feetaway from each other.

Ghez's research is supported by theNational Science Foundation and the Packard Foundation.



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