Two Black Holes Merging

November 20, 2002
 Two black holes seen merging in millions of years

In a very bright galaxy 400 million light-years away, two black holes are drifting toward each other and in millions of years will merge with an eruption of energy and a burst of gravitational waves that could warp the very fabric of space, astronomers said yesterday.

The scientists said the Chandra X-ray Observatory has found the first evidence that two immense black holes can coexist in the same galaxy and they are moving toward each other for an eventual merger.

The double black holes were found in a bright, highly active galaxy known as NGC6240, about 400 million light-years from the Earth.

Astronomers studied NGC6240 because it produced unexplained bursts of X-rays that appeared to come from one of two nucleuses at the galactic center. Images collected by radio, infrared and optical observations showed two bright spots, but did not pinpoint the origin of the X-rays.

When Chandra, with its sensitive X-ray detectors, focused on the nucleuses, astronomers hoped it would tell them whether either of the two points of activity were black holes. “Much to our surprise, we found that both were active black holes,” Stefanie Komossa of the Max Planck Institute in Germany, said in a statement.

Finding two black holes in one galaxy, said Miss Komossa, “supports the idea that black holes can grow to enormous masses in the centers of galaxies by merging with other black holes. Guenther Hasinger, also of Max Planck, said the Chandra images captured the unmistakable markings of two black holes, high-energy photons swirling around the dense black hole centers and X-rays spewing out from iron atoms being pulled into the center at a high rate of speed.

Miss Komossa and Mr. Hasinger are co-authors of a study submitted for publication in the Astrophysical Journal Letters. The two black holes in NGC6240 are now about 3,000 light-years apart and are expected to merge sometime in the next few hundred million years, the researchers said.

The merger will be accompanied by an eruption of radiation and a burst of gravitational waves that will spread throughout the universe, causing ripples in the fabric of space, the astronomers said. They said the gravitational ripples could cause minute changes in  the distance between any two points in the universe.

In another study, French and Argentine astronomers said that observations by the Hubble Space Telescope and ground-based telescopes have detected a stellar black hole streaking across the Milky Way at 250,000 miles an hour.

A companion star is being dragged along and slowly devoured by the black hole, according to scientists at the French Atomic Energy Commission and the Institute for Astronomy and Space Physics in Argentina.

The astronomers said the stellar black hole may have been created by an exploding star in the inner disk of the Milky Way. The black hole is 6,000 to 9,000 light-years away, the researchers said. A report on the observations appeared yesterday in the journal Astronomy and Astrophysics.

A black hole is a point in space that is so dense with matter that its gravitational field will not let anything “not even light” escape. Stellar black holes, equal to 3.5 to about 15 solar masses, can be formed by the collapse of a single massive star.

But galactic black holes, such as those in NGC6240, are much larger, equal perhaps to millions of solar masses, and are usually at the center of galaxies.