Twin giant black holes discovered

Nov. 19, 2002 at 10:00 PM
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HUNTSVILLE, Ala., Nov. 19 (UPI) -- For the first time, two supermassive black holes have been found orbiting each other at the center of a distant galaxy, NASA astronomers reported Tuesday.

The celestial monsters, captured in an image by the orbiting Chandra X-ray telescope, are seen locked in a gravitational dance of death, a condition that leads inevitably to a collision that releases gigantic amounts of radiation as well as the mysterious and as-yet-undetected phenomenon called gravitational waves.

Astronomers focused Chandra's sensitive X-ray imaging cameras on the center of a bright galaxy called NGC 6240, located some 400 million light-years from the Milky Way galaxy.

A light-year is the distance that light, traveling at about 186,000 miles per second, covers in one year -- about 6 trillion miles.

So, an image from a galaxy some 400 million light-years distant means the event took place 400 million years ago.

Previous X-ray images collected by other observatories had shown NGC 6240's central region produces X-radiation -- a signature of supermassive objects. In addition, other images of the region -- taken in the radio, infrared and visible light ranges of electromagnetic radiation -- showed two bright nuclei, but the nature of the twin sources remained a mystery.

The new Chandra images have confirmed that NGC 6240 actually contains two giant black holes that are sucking in material from their surroundings with their enormous gravitational attraction.

"With Chandra, we hoped to determine which one, if either, of the nuclei was an active supermassive black hole," said Stefanie Komossa, of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

"Much to our surprise, we found that both were active black holes," said Komossa, lead author of a paper on NGC 6240 that is scheduled to appear in an upcoming issue of the Astrophysical Journal Letters.

NGC 6240 has been the subject of intense study because it was created by the merger of two smaller galaxies. Galactic collisions tend to kick up gigantic volumes of interstellar gas and dust, which tend to condense into new stars at a relatively rapid rate. However, this same material also makes such hybrid galaxies difficult to observe, astronomers said, because it smears visible light.

But Chandra's X-ray telescope -- which circles Earth in an elliptical orbit that ranges from about 6,000 to 86,000 miles -- can penetrate the obscuring dust and gas.

"The breakthrough came with Chandra's ability to clearly distinguish the two nuclei, and measure the details of the X-radiation from each nucleus," said Guenther Hasinger, also of the Planck Institute and the paper's co-author.

"These cosmic fingerprints revealed features characteristic of supermassive black holes -- an excess of high-energy photons from gas swirling around a black hole, and X-rays from fluorescing iron atoms in gas near black holes," he said.

The discovery reinforces current theories of supermassive black hole formation, said Ray Villard, a professor of astronomy at Johns Hopkins University in Baltimore, Md.

"The Hubble Space Telescope has shown that most if not all galaxies harbor supermassive black holes," Villard, also the news director for the Space Telescope Science Institute, told United Press International. "It stands to reason that if two galaxies merge, the black holes at their respective centers would be drawn closely together."

Many black holes are created by the explosions of extremely large stars, called supernovae, which collapse stellar remnants into an incredibly dense ball of matter. But the supergiants located at the centers of galaxies, which are hundreds or even thousands of times larger than the black holes created by supernovae -- and millions of times the mass of our sun -- seem to be grown by a different process.

Though no one knows for sure, one possibility is that supermassive black holes are grown by smashing together the smaller black holes, errant stars and planets, and dust and gas that naturally inhabit galactic hearts.

"The detection of a binary black hole supports the idea that black holes can grow to enormous masses in the centers of galaxies by merging with other black holes," Komossa said.

"This is important for understanding how galaxies form and evolve."

Over the next several hundred million years, she explained, the black hole twins -- which circle each other at the relatively close distance of 3,000 light-years -- will drift together and eventually merge into an even larger black hole, setting off a titanic event that will affect the fate of the entire galaxy.

Villard said on an even longer timeframe, a similar outcome might await the Milky Way. About 6 billion years from now, he explained, our galaxy and the neighboring Andromeda galaxy -- currently separated by about 2 million light-years -- will merge, just as the two galaxies that comprise NGC 6240.

"It would be a wonderful time to be an astronomer," he said, because the night sky would be far brighter and more active than it is now, full of new, brilliant blue star clusters and more frequent supernovae.

Yet that sky show would be only a prelude to a much bigger, and possibly lethal, event: the merger of the two galactic supermassive black holes.

"The collision might ignite a quasar," he said.

Quasars, short for quasi-stellar objects, are the brightest objects in the universe. They emit more light in an instant than the sun does over its entire 10-billion-year lifetime.

The closest known quasar is about 600 million light-years away. But if a quasar were ignited at the center of the Milky Way -- or the center of the combined new galaxy -- its radiation might be powerful enough to endanger any living things within the galactic borders.

"If the Milky Way had a quasar," Villard said, "we'd all have to wear SPF (sun protection factor) 5,000."

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(Reported by Phil Berardelli, UPI Deputy Science & Technology Editor, in Washington)

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(Editors: UPI Photo # WAX2002111901 available)

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