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Ribbon of Gas May Be Clue to Power of Galaxy

Times Science Writer

A ribbon of gas more than 90 trillion miles long has been discovered streaming toward the center of the Milky Way galaxy, and it may provide the answer to the baffling question of what powers the galaxy.

An international team of scientists announced the discovery here Tuesday amid claims that the ribbon of gas offers dramatic new evidence that a giant black hole, more massive than a million suns, lies at the heart of the Milky Way.

Many astronomers believe that an enormous black hole provides the energy that powers the Milky Way, driving billions of stars in giant orbits around the galactic center as it pulls in more and more mass.

Such a black hole would be so powerful that it would already have devoured most of the mass near the galactic center, which is about 30,000 light years away from Earth, according to Prof. Paul Ho of Harvard University, who announced the discovery during a meeting of the American Astronomical Society.

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Ho, who is also with the Harvard-Smithsonian Center for Astrophysics, said many scientists now believe a giant black hole at the center of the Milky Way serves as the “engine” that powers the galaxy. But it must constantly gobble up mass to drive the system, and if it has already devoured nearly everything in its neighborhood, where does it get its fuel?

“How do you feed the central engine?” Ho asked during a press conference.

Some scientists believe an occasional passing star is sucked into the black hole, but to serve as the galaxy’s engine the black hole would need a fairly constant flow of matter. So far, no one has been able to figure out where it comes from.

Ho and his colleagues believe they may have found the answer in the discovery of a long stream of molecular gas that is feeding into the galactic center. He said the gas appears to be supplying matter to a giant shell of gas and dust that surrounds the galactic center. Material from the shell falls into the black hole, Ho theorized, and is replaced by mass from the ribbon of gas.

“It’s the first time that we have seen evidence that suggests such a phenomena,” he said. Ho’s team includes scientists from the Max Planck Institute for Extraterrestrial Physics in Munich, the University of Cologne and the Massachusetts Institute of Technology.

Black holes are believed to be created when a star collapses, and they are so dense that not even light can escape their gravitational pull. Thus no one has ever seen a black hole, although an increasingly large number of scientists now believe they exist.

Continue to Grow

An “ordinary” black hole might be no larger than a few miles in diameter, yet as massive as the sun. According to theory, black holes devour nearby matter--including stars--and they continue to grow as they feed on their neighbors.

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Some could become huge, influencing the movement of distant stars and possibly playing a key role in the formation of galaxies.

Using the Very Large Array of radio telescopes in the New Mexican desert, the scientists created a radio map of the stream of gas. The stream is at least 15 light years long (90 trillion miles) and Ho suspects that it is actually longer, stretching out to a giant cloud of molecular gas more than 25 light years away from the galactic center.

He theorized that an exploding star sent shock waves through the cloud, causing the gas to form into a long ribbon with enough density for the black hole to suck it in toward the center of the galaxy.

Such a system would provide an almost endless source of fuel.

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Ho described the stream as a “rich collection of molecules” that is probably fairly young in astronomical terms--possibly only 10,000 years old. Presumably, the process could be repeated over and over with new streams drawing on the immense interstellar clouds that lie about 30 light years from the galactic center.

Ho said a team of astronomers at the University of Tokyo’s Nabeyana Radio Observatory had also detected the stream of gas in an independent program.

Several scientists familiar with Ho’s work said they do not doubt the reliability of the data.

Not All Agree

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But not everyone agreed with Ho’s interpretation.

Kenneth Brecher, an astrophysicist with Boston University, has long argued against black holes as the “engines” that power galaxies.

For a black hole to be the engine of the galaxy, it would need to be as massive as a million suns, which Brecher says is highly unlikely. Any large source of gravity, including a cluster of stars, could supply the energy needed to run the galaxy, he added. Optical telescopes cannot reveal whether Brecher or Ho is right, because visible light from even a million suns cannot get through the cloud of dust and gas that surrounds the galactic center.

Source of Gamma Rays

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Most astronomers are leaning heavily toward the black hole theory, however, partly because of a rapid accumulation of circumstantial evidence that is consistent with the presence of a black hole. An enormous black hole gobbling up matter would send out some signals, such as gamma rays, and previous research revealed that something near the galactic center is a powerful source of gamma rays.

Other radiation, such as X-rays, also flows from the galactic center consistent with the violent activity that would be typical of a huge black hole.

Thus, while some evidence supports the contention that at the heart of every galaxy is a massive black hole, no one has been able to prove it. But scientists attending the meeting here said the circumstantial evidence, now bolstered by Ho’s discovery of the ribbon of molecular gas as revealed by radio emissions, is becoming increasingly compelling.


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