A megamerger in space: Two black holes collide

LIKE a pair of cosmic sumo wrestlers, two supermassive black holes in a galaxy 4 million light-years away are warily approaching each other in preparation for a titanic collision that will shake the very fabric of space-time.

And Gunther Hasinger has a front-row seat.

Using the Chandra X-Ray Observatory, Dr. Hasinger and his colleagues have collected images of the pair, the first such images of galactic black holes merging.

The merger is the result of a collision between two large galaxies, identified collectively as NGC6240. The Chandra observation could help astronomers account for the presence of massive black holes at the center of many galaxies, including our own Milky Way. It also provides a window on how galaxies have formed and evolved over the past 15 billion years. Such events play a key role in this process, affecting not only the size of galaxies but their rate of star formation.

"Everyone has assumed that if the galaxies merge, then in the end the black holes will also merge," Hasinger says. "Now, for the first time, we've found two active black holes merging." The team, led by Stefanie Komossa of the Max Planck Institute of Extraterrestrial Physics in Garching, Germany, announced its results yesterday at a NASA briefing in Washington.

The work represents "an important milestone" in understanding the role played by some of the most bizarre objects in the universe, according to Harvey Tananbaum, director of the Chandra X-Ray Center at the Smithsonian Astrophysical Observatory in Cambridge, Mass.

Black holes are objects so massive that light fails to travel fast enough to escape their intense gravity. They can form from individual stars that end in giant explosions known as supernovae.

On Monday, astronomer Felix Marabel announced he had detected a small black hole speeding through the plane of the Milky Way. The observation - based on the motions of a small star orbiting the black hole - suggests that the black hole is ripping through interstellar space four times as fast as the surrounding stars The black hole - from 3.5 to 15 times as massive as the sun - is thought to have gotten its kick from a supernova explosion.

But the black holes in the center of galaxies are in a class unto themselves. While they can't be viewed directly, they make their presence known by their gravitational effect on surrounding material and by the radiation representing the last SOS from gas, dust, and other objects as they fall into the black hole's grasp.

The objects Dr. Komossa's team has been tracking pack 10 million to 100 million times the sun's mass into objects with the diameter of our solar system. The two black holes currently are 3,000 light-years apart - roughly the distance between Earth and the Trifid Nebula in the constellation Sagittarius in our own galaxy. And the process is achingly slow. The final merger is expected to take place in 100 million years.

When they do, their collisional shudder will ripple through the cosmos in the form of gravity waves, a phenomenon predicted by Albert Einstein's theory of general relativity.

Unless someone is up close to the collision, it won't rattle any dishes. Astronomers calculate than when the Andromeda galaxy collides with our own galaxy in about 4 billion years, the galaxies' black holes will merge. As seen from Earth, the center of the Milky Way, currently obscured by dust, will glow as brightly as the full moon. Gravity waves from the collision would be expected to shift the Earth a few centimeters from its pre-collision orbit, Hasinger notes.

Yet no gravity waves from any source have yet been detected, although physicists are working on it. They have built a pair of gravity-wave detectors on Earth - one in Louisiana and one in Washington State. The project, called LIGO, began taking data in September.

By 2012, astronomers hope to send a gravity-wave detector into space. The project, known as LISA, is being undertaken by the National Aeronautics and Space Administration and the European Space Agency.

The interest in mergers of black holes and their parent galaxies stems from a broader effort to understand the evolution of stars and galaxies over the universe's history. Astronomers have noted that merging galaxies, including NGC6240, undergo a burst of intense star formation as the gravitational maelstrom from the collision triggers clumping of gas and dust into proto-stars. These collisions also could help explain why a galaxy's central black hole seems to automatically scale itself to a size appropriate for the galaxy it inhabits. Smaller galaxies with fewer collisions are likely to have smaller central black holes, Hasinger says.

Ultimately, this team's data and followup observations could help round out the picture astronomers are trying to paint of the universe's early days. "The first star in the universe also may have ended as the first black hole," Hasinger speculates. Its gravitational effect on surrounding matter could have helped triggered star formation nearby, spawning baby galaxies.

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