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Black Holes Are "Green," X-Ray Study Says

John Roach
for National Geographic News
April 24, 2006
 
Supermassive black holes are actually "green," scientists announced today as they described a new study on the energy efficiency of black holes.

If cars were as fuel efficient as these black holes, researchers say, the vehicles could theoretically travel over a billion miles (1.6 billion kilometers) on a gallon of gasoline.

"In anyone's book, that would be pretty green," Steve Allen, an astrophysicist with Stanford University in Palo Alto, California, and lead author of the study, said during a telephone briefing with reporters.

Using NASA's Chandra X-ray Observatory (related images from Chandra), scientists hope to provide insight into how older, supermassive black holes at the centers of elliptical galaxies generate energy and affect their environment.

According to the study, most of the energy released by matter falling toward these supermassive black holes is in the form of high-energy jets traveling near the speed of light away from the black hole.

These jets create bubbles thousands of light-years across in the hot gas in galaxies. The energy supplied to these bubbles keeps the hot gas from cooling, thereby preventing billions of new stars from forming.

This should place limits on the growth of the largest galaxies, scientists said.

"In an environmental sense, the black holes are actually preventing galactic sprawl from taking over the neighborhood," Kimberly Weaver, an astrophysicist with the NASA Goddard Space Flight Center in Greenbelt, Maryland, said at the briefing.

Weaver, who was not part of the study team, added, "This is another way that these black holes could be considered green."

Not So Boring

Black holes are places in space where the force of gravity is so strong that nothing, not even light, can escape. Supermassive black holes are found at the center of every galaxy in the universe.

Allen and his colleagues studied nine older supermassive black holes in nearby galaxies that have received less scientific attention than quasars, a younger type of supermassive black hole.

Christopher Reynolds, an astronomer and study co-author at the University of Maryland in College Park, explained that quasars are considered more exciting because the material that falls into them is converted into bright light.

"This is what makes them powerful beacons of light that can be viewed across most of the observable universe," he said during the briefing.

The older supermassive black holes, Reynolds and Allen had observed, were not generating as much radiation—light, x-rays, et cetera—as would be expected from the amount of material falling toward them.

The team's study of these black holes, however, revealed these black holes are "extremely efficient not at producing light but at driving powerful jets," Reynolds said.

"So we've shown that black holes never seem to lose their efficiency at converting matter into energy, they just put energy out in different ways in different phases."

According to Reynolds, these older black holes, each about a billion times more massive than our sun, release about a thousand times more energy as jets rather than as light.

Measuring Efficiency

To find out how efficient the supermassive black holes are, the researchers first used Chandra images of the central regions of elliptical galaxies to estimate the amount of fuel available for the black hole.

The team then used Chandra images to measure the size of the bubbles created by the jets and the jet power.

"So putting these results together—the fueling rates and the jet power—we measure an efficiency for these black-hole engines that goes far beyond any engine we are used to here on Earth," Allen said.

According to Allen, the study represents the first direct measurement of black hole fuel efficiency.

Exactly why these older, supermassive black holes are so efficient at making these jets will require further study, Reynolds added.

"But that's great. This is just part of the learning process by which we learn how these fascinating complex objects work."

A paper on this research will appear in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

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