[Nasional-e] 2 black holes to collide (but not anytime soon)

[Ambon]

2 black holes to collide (but not anytime soon)
Warren E. Leary The New York Times  Thursday, November 21, 2002

WASHINGTON Two giant black holes have been found at the center of a galaxy
born from the joining of two smaller galaxies, and they are drifting toward
a cataclysmic collision that will send ripples throughout the universe many
millions of years from now, scientists said.
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The detection of the supermassive black holes — collapsing objects so dense
that their gravity draws in all material around them, including light — is
the first definitive evidence that two of them can exist in the same galaxy.
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These particular black holes, found by a team of researchers using the
orbiting Chandra X-Ray Observatory, are circling each other in a Mephisto
waltz that will lead to their merging in several hundred million years. That
joining, astronomers said, will result in a monumental release of radiation
and gravitational waves that should stretch across the universe.
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‘‘These gravitational waves will spread out to produce ripples in the fabric
of space,’’ a member of the research team, Gunther Hasinger, said at a
briefing for reporters Tuesday coordinated by the National Aeronautics and
Space Administration.
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Eventually, those ripples will hit Earth’s galaxy and cause infinitesimal
wobbling in all matter, though it would be far too tiny to be noticed by
humans.
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Hasinger, an astrophysicist at the Max Planck Institute of Extraterrestrial
Physics near Munich, said that previous observations of NGC 6240, the galaxy
with the two black holes, had detected only two bright center regions. But
the Chandra observatory’s ability to make high-resolution observations of
X-ray emissions identified the sources of those radiation bursts as black
holes, he said.
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Stefanie Komossa, a Max Planck astronomer who is co-author of a paper on the
discovery that is to be published in Astrophysical Journal Letters, said:
‘‘This is the first time we have ever identified a binary black hole. This
is the aftermath of two galaxies that collided sometime in the past.’’
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NGC 6240 is 400 million light-years from Earth. Observations of it with the
Hubble Space Telescope and other instruments had shown two bright knobs of
light but no detail at the galaxy’s center, Komossa said.
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Astronomers hoped to use Chandra, which has revolutionized X-ray astronomy
since its launching in 1999, to tell which of the bright spots might be a
supermassive black hole. ‘‘Much to our surprise,’’ Komossa said, ‘‘we found
that both were active black holes.’’
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The observations showed that the black holes were each about the size of the
inner solar system, a distance stretching from the sun to Mars, and were
circling each other at a distance of 3,000 light-years.
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Astronomers said that the merging of two similar-size galaxies into NGC 6240
was also a prelude to the future of Earth’s galaxy, the Milky Way, composed
of hundreds of millions of stars, including the sun. Many scientists
maintain that most galaxies, including the Milky Way, have giant black holes
with the mass of millions of stars at their centers. In about 4 billion
years, astronomers maintain, the Milky Way and the nearby Andromeda galaxy
will collide and merge, fusing their black holes into one.
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‘‘We’re seeing our own future,’’ Steinn Sigurdsson of Pennsylvania State
University, an astronomer who was not a member of the study team, said at
the briefing Tuesday.
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Hasinger noted that humans on Earth would not have to worry about this
galactic collision: They will not be around. The sun is expected to blow up
into a nova in 3 billion years, and perhaps then collapse to form a small
black hole of its own, he said.
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Hasinger said black-hole mergers as seen in NGC 6240 were probably common
throughout the universe. Many quasars, distant bright objects that emit huge
amounts of X-rays and other radiation, may be examples of black-hole
mergings, he said. New space-based detectors, like NASA’s planned Laser
Interferometer Space Antenna, should be able to detect gravity waves from
these events that could help determine how black holes are formed.
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Komossa said it was possible that more than two massive black holes could
merge, creating even larger ones with different characteristics.
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Scientists have so far identified two types of black holes: giant ones with
the masses of millions of stars that lurk at the core of galaxies, as in NGC
6240 and the Milky Way, and smaller stellar black holes, the collapsed
remnants of big stars with masses three times to 15 times that of the sun.
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In another study, published this week in the journal Astronomy and
Astrophysics, Scientists with the French Atomic Energy Commission and the
Institute for Astronomy and Space Physics in Argentina said that
observations by the Hubble Space Telescope and ground-based telescopes had
detected a stellar black hole streaking across the Milky Way at
approximately 400,000 kilometers an hour (250,000 miles an hour).
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The report said a star was being dragged along with the black hole and was
slowly being consumed by it. The scientists said that this black hole, from
6,000 to 9,000 light-years from Earth, might have been created by an
exploding star in the inner disk of the Milky Way.