Black Holes Set to Collide

Astronomers Foresee Enormous Collision of Two Black Holes
By WARREN E. LEARY


WASHINGTON, Nov. 19 — Two giant black holes have been found at the center of a galaxy born from the joining of two smaller galaxies and are drifting toward a cataclysmic collision that will send ripples throughout the universe many millions of years from now, scientists said today.

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.

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.

"These gravitational waves will spread out to produce ripples in the fabric of space," one member of the research team, Dr. Gunther Hasinger, said at a NASA briefing for reporters today.

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.

Dr. Hasinger, an astrophysicist at the Max Planck Institute of Extraterrestrial Physics, near Munich, said 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.

Dr. 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."

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, Dr. Komossa said. 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," Dr. Komossa said, "we found that both were active black holes."

The observations showed that the big black holes were each about the size of the inner solar system that includes Earth, a distance stretching from the Sun to Mars, and were circling each other at a distance of 3,000 light-years.

Astronomers said 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 believe that most galaxies, including the Milky Way, have giant black holes with the mass of millions of stars at their centers. In about four billion years, astronomers believe, the Milky Way and the nearby Andromeda galaxy will collide and merge, fusing their black holes into one.

"We're seeing our own future," Dr. Steinn Sigurdsson of Pennsylvania State University, an astronomer who was not a member of the study team, said at the briefing today.

Dr. 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 three billion years, and perhaps then collapse to form a small black hole of its own, he said.

Dr. Hasinger said black-hole mergers as seen in NGC 6240 were probably common throughout the universe, with several each year in areas that can be viewed from Earth. 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.

Dr. Komossa said it was possible that more than two massive black holes could merge, creating even larger ones with different characteristics.

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 3 to 15 times that of the Sun.

In another study, published this week in the journal Astronomy and Astrophysics, French and Argentine astronomers said observations by the Hubble Space Telescope and ground-based telescopes had detected a stellar black hole streaking across the Milky Way at some 250,000 miles an hour.

The report said a companion star was being dragged along with the black hole and was slowly being consumed by it.

Scientists with the French Atomic Energy Commission and the Institute for Astronomy and Space Physics in Argentina said 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.

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