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Black holes pair up for double whammy

Though these black hole pairs had been predicted theoretically, only a handful had been observed so far.
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What's worse than one giant black hole waiting to swallow up everything nearby? Try two supermassive black holes packing a galactic double whammy.

A team of astronomers recently discovered 33 sets of double black holes in distant galaxies. Though these black hole pairs had been predicted theoretically, only a handful had been observed so far.

"[The findings] show that dual supermassive black hole systems are much more common than previously known from observations," said researcher Julia Comerford, an astrophysicist at the University of California, Berkeley.

Comerford presented the discovery Monday at the 215th meeting of the American Astronomical Society in Washington, D.C.

Scientists think that most galaxies, including our own Milky Way, host supermassive black holes lurking at their centers. So pairs of giant black holes are thought to result when two galaxies collide and merge together. Each galaxy's black hole would gradually spiral in toward the center of the newly created fusion galaxy, and the two black holes would attract one another in a gravitational tug of war.

In fact, this fate is predicted for the supermassive black hole inside our own galaxy. The Milky Way is headed toward an eventual collision with the neighboring Andromeda galaxy, and their two black holes will likely join in just this way.

Most of the newly discovered pairs were found through the DEEP2 Galaxy Redshift Survey, a survey of 50,000 galaxies observed with the Deep Imaging Multi-Object Spectrograph on the 10-meter (400-inch) Keck II Telescope atop Mauna Kea in Hawaii.

The astronomers detected their pairs by noting their relative motion to each other. When something moves away from an observer, its light appears redder, or red-shifted (this shift in wavelength is known as the Doppler effect). And when an object moves toward an observer, its light appears bluer.

So the researchers measured the relative velocities of apparent black holes inside distant galaxies and found some sets that appeared to be very close to each other — about 3,000 light-years apart, or one-eighth the distance from the sun to the center of the Milky Way — indicating they were gravitationally-bound in pairs.