Cosmic Couple: One galaxy, two gravitational beasts

In a single galaxy, two massive black holes are spiraling toward each other in a gravitational dance that will end in a few hundred million years, when the black holes merge, astronomers report.

They had good reason to suspect there’d be such pairs. Accumulating evidence has revealed black holes in galactic centers and mergers of galaxies. So, it follows that some galaxies ought to have two black holes. “It was starting to become a little embarrassing that there was actually so little evidence of any [such] galaxies,” says Roeland van der Marel of the Space Telescope Science Institute in Baltimore.

The new finding comes from NGC 6240, an extraordinarily bright galaxy only 400 million light-years from Earth. This galaxy is the product of an ongoing merger of two galaxies, a process characterized by distorted shapes and flailing loops and tails. The galaxy’s relative proximity made it easy for the Chandra X-Ray Observatory, which has been orbiting Earth since 1999, to discern the two black holes in its center.

“It was a surprise,” says Stefanie Komossa of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. She and her colleagues were drawn to NGC 6240 by its unusual properties, including its infrared brightness, strong X-ray emissions, huge concentrations of gas, and fast-moving stars. They conjectured that a single black hole could partially explain such characteristics.

However, studying NGC 6240 has been tricky. “It’s very difficult to actually look at the very center of this galaxy because it is strongly obscured by clouds of gas and dust,” says Komossa. Images obtained by the Hubble Space Telescope show two distinct bright spots, hinting at a binary black hole system. But it isn’t possible from these visible-light images to tell whether the spots arise from the center or surface of the galaxy.

With its X-ray vision penetrating the galaxy’s center, Chandra has settled that issue. “X rays allowed [us] for the first time to zoom into the center of this galaxy,” says Komossa. “These two very massive black holes emerged.”

The finding, to be reported in an upcoming Astrophysical Journal Letters, supports the view that just as galaxies can grow by merging, so can black holes.

The X-ray emissions also indicate that the black holes are actively devouring interstellar matter around them. “Most galaxies have black holes that are inconspicuous,” says van der Marel, who took the Hubble images of NGC 6240. He says that Komossa’s team was fortunate to find a galaxy in which two massive black holes are both radiating large amounts of X rays.

As the two giant black holes in NGC 6240 bridge their 3,000–light-year gap and merge over millions of years, they will release intense radiation. They’ll also emit powerful gravitational waves. These waves produce ripples in space and cause distances between points to fluctuate. After the Laser Interferometer Space Antenna, or LISA, is launched in 2008, it should detect these types of waves from black holes already merging.

Meanwhile, Komossa says her team will “look much deeper” into NGC 6240 and also search for binary black holes in similar galaxies.

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