Black Holes Bound to Join Forces

Researchers finally have strong evidence for the existence of a binary black hole system, a long-theorized result of galactic mergers that features two black holes orbiting around each other at the center of large galaxy.

The black holes are expected to merge in what astronomers figure would be one of the most energetic events in the universe.

The new evidence comes from a study of the light signatures of 17,500 quasars ? extremely bright features at the center of galaxies, thought to be powered by black holes ? taken by the Sloan Digital Sky Survey and detailed in the March 5 issue of the journal Nature.

It has long been accepted that most large galaxies harbor black holes at their centers. Astronomers also think that mergers between galaxies are common ? our own Milky Way is on a collision course with our nearest neighbor, the Andromeda galaxy (though this merger won't happen for several billion years).

By combining these tenets of astrophysics, it's a logical leap to the eventual merger of each galaxy's black hole.

"They should merge, and we expect them to," said study team member Tod Lauer of the National Optical Astronomy Observatory (NOAO) in Tucson, Ariz.

Best evidence

Lauer likened galaxies to fluffy masses and the black holes at their centers to rocks ? eventually through interactions with the fluff (the stars of the galaxy), the black holes lose orbital energy and fall to the center of the newly-merged galaxy.

But once they've migrated to the center, interactions with stars are rare, "and the black holes orbit each other in a small tightly bound orbit, unable to loose enough momentum so that they could actually merge," Lauer explained.

It was at this stage that Lauer and study leader Todd Boroson, also of the NOAO, think they detected the signature of two separate black holes in the spectrum of the quasar known as SDSS J153636.22+044127.0.

The black holes appear to be separated by a mere 0.1 parsec, 13 times shorter than the distance between our sun and its nearest neighbor, Alpha Centauri. The researchers roughly estimated the masses of the pair, coming up with 10 million solar masses for the smaller black hole and a little less than 1 billion solar masses for its larger partner. They also estimated that the black holes orbit each other about every 100 years.

Previous studies, including one of the galaxy NGC 6240, have shown potential evidence of black holes heading for a merger, "but the case presented by Boroson and Lauer is special because the pairing is tighter and the evidence much stronger," wrote astronomer Jon Miller of the University of Michigan in an accompanying Nature analysis. Miller was not involved in the new study.

"We think we have the best one so far," Lauer told

To catch in the act

While this evidence shows possible binary black holes squeezed closer together than any yet found, Boroson and Lauer still need solid proof that this is a binary system.

"The issue is that this is a good candidate for a binary, but we haven't proved it beyond doubt," Lauer said.

Once the black holes have reached the gravitational center of the merged galaxy and come as close as they can through interactions with the surrounding stars, they need an extra push to fully merge. If gas later collects at the center of the galaxy, it could absorb some of the remaining orbital momentum of the black holes and provide that extra oomph to push them into a merger.

Black holes that have reached the point of merging are expected to give off gravitational waves, or ripples in the fabric of space-time predicted by Einstein's General Relativity. Scientists hope that the Laser Interferometer Space Array (LISA), currently in development and designed to detect these waves, will be able to catch black holes actually in the act of merging.

In the meantime, Lauer and Boroson will continue observing their quasar to see if changes in the spectrum over time (as the black holes orbit one another) further support the existence of a binary black hole and rule out the possibility that they are two separate objects super-imposed. Lauer thinks this is unlikely because "the spectrum is difficult to explain with two different objects."

The paucity of signatures of black hole mergers and binary black holes so far could mean that these are much rarer events than scientists had thought.

"We looked through 17,000 of these and found this one beast," Lauer said. "They should be very common."

But it's possible that LISA could provide a wealth of evidence once it gets off the ground. It's also possible that binaries are only detectable when actively accreting gas and that most reside in so-called normal galaxies that are not in an actively accreting quasar phase, Miller noted in his editorial.