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Black hole triplets spotted

By Ker Than
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( -- The discovery of three distant supermassive black holes in proximity to one another is giving astronomers a glimpse into the chaotic early years of the universe.

Known as quasars, these incredibly bright objects are thought to be powered by gas falling into enormous black holes situated in the centers of galaxies. Although smaller than our solar system, a single quasar can outshine an entire galaxy of a hundred billion stars.

Roughly 100,000 quasars have been observed in recent years, some of them double quasars. But this is the first time that three quasars have been found so near one another. The three quasars are separated by about 100,000 to 150,000 light years -- about the width of our Milky Way.

"Quasars are extremely rare objects. To find two of them so close together is very unlikely if they were randomly distributed in space," said study leader George Djorgovski, an astronomer at Caltech. "To find three is unprecedented."

The discovery, made using the European Space Observatory's Very Large Telescope and the W.M. Keck Observatory Hawaii, was reported here today at the 209th meeting of the American Astronomical Society. A science paper on the research has been submitted to Astrophysical Journal Letters.

One of the quasars in the triplet, called LBQS 1429-008, was discovered in 1989 by a team of astronomers led by Paul Hewett of the Institute of Astronomy in Cambridge, England.

Hewett's team later spotted what appeared to be a faint companion to their quasar, but they dismissed it as a visual illusion created by gravitational lensing, whereby light traveling towards Earth from distant galaxies is bent by the gravity of massive objects -- such as star clusters, dark matter clumps or other galaxies -- in their path. Sometimes, this can create mirror images of a galaxy in one or more different places in the sky.

But the new observations have revealed a third, even fainter companion to Hewett's quasar duo and, equally important, the lack of any galaxy that could possibly serve as a lens. This makes it highly unlikely that gravitational lensing is responsible, the researchers say.

"We just could not reproduce the data," said study team member Frederic Courbin of Ecole Polytechnique Federale de Lausanne in Switzerland. "It is essentially impossible to account for what we see using reasonable gravitational lensing models."

The team also observed small differences in the properties of the three quasars that are best explained if the quasars are physically distinct objects.

Double quasars are thought to form when two galaxies, each containing their own supermassive black holes, merge together. While the amalgamation of two black holes is thought to just create a larger black hole, the merger of three black holes is expected to be more violent and interesting.

"Three is so much better than two because the dynamics of three gravitationally interacting bodies is chaotic, as opposed to the much more regular motion of two bodies simply orbiting each other," said Frederic Rasio of Northwestern University in Illinois.

Rasio was not involved in the triple quasar observations, but his team has done recent theoretical work showing that interactions between three supermassive black holes might occur as frequently as a few times per year within the observable universe.

Rasio's work also predicts that triple quasar encounters could eject one or more of the bright objects straight out of the host galaxy. These "naked" quasars are then destined to wander alone through the universe.

The triple quasar discovery provides astronomers with a rare glimpse into an early, and much more chaotic, period of the universe. The quasar trio is located about 10.5 billion light years away in the Virgo constellation.

This means light from those quasars that astronomers now see were emitted when the universe was just over 3 billion years old. During that early epoch, the universe was a much smaller space; galaxies were also smaller and collided much more frequently.

"Quasars are believed to be powered by gas falling into supermassive black holes," Djorgovski said. "This process happens very effectively when galaxies collide or merge, and we are observing this system at the time in the cosmic history when such galaxy interactions were at a peak."

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Three arrows indicate the positions of three quasars separated by about 100,000 to 150,000 light years -- about the width of our Milky Way.



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