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Triple quasar hints at violent past

January 9, 2007 By Alexandra Witze This article courtesy of Nature News.

Colliding galaxies in the early Universe produced dance of superbright objects.

Astronomers have found a new record-breaker: a triplet of quasars.

Quasars are extremely bright and ancient objects that pour out colossal amounts of radiation as much as a trillion Suns, or even more. They are the superbright signs of hot matter falling into black holes at the hearts of distant galaxies.

Quasars are also so rare that they usually are found as loners. Of the 100,000 or so quasars known, only about 100 of them come as pairs. Now scientists have spotted the first system containing three quasars.

The discovery hints at the dramatic events that shaped the Universe in its early years, says George Djorgovski, an astronomer at the California Institute of Technology in Pasadena who led the research team. Back then, the Universe was smaller and galaxies slammed into each other more often. So the massive black holes at the galaxies' centers interacted with each other, creating powerful quasars.

"The punchline is that these interactions are quite common," says Frederic Rasio, a theoretician at Northwestern University in Evanston, Illinois, whose work could help explain how the triplet came to be.

Early days

The punchline is that these interactions are quite common.
Frederic Rasio
Northwestern University
The newfound system lies more than 10 billion light-years (90 billion trillion kilometres) from Earth. That means our view of it comes from early cosmic days; the Universe is thought to be 13.7 billion years old.

Astronomers first spotted this particular quasar system in 1989, but thought it was just a single quasar whose light had been bent by intervening galaxies so as to make it appear as two. Later, other scientists realized it was at least two distinct quasars.

Now, Djorgovski's team has shown that it is really a quasar mnage ; trois. Using the Keck telescopes in Hawaii and the European Southern Observatory's Very Large Telescope in Chile, the team confirmed that there is a third, fainter, quasar.

There's just a 1-in-20-quadrillion probability that three quasars could have formed so close together by chance, Djorgovski told reporters at a meeting of the American Astronomical Society in Seattle, Washington on 8 January.

Instead, he says, astronomers are probably seeing the first steps of an intricate cosmic dance, in which galaxies slam together and the black holes at their hearts begin to interact.

Three's company

At the meeting, Rasio presented findings that could help explain how the triple quasar formed.

His team has created computer simulations of what might happen when a pair of black holes encounters a third. The work shows how, over the course of many millions of years, the black holes zip around each other in complex looping orbits. That's the stage that Djogorvski sees today a moment when the black holes are interacting with each other, producing three visible quasars.

But eventually the gravitational interactions become unstable, and the black holes slingshot each other out of the neighbourhood. Errant black holes go zooming outwards at speeds reaching thousands of kilometres per second - or more.

"At the end, nothing is left," says Rasio. "They kick each other out of the centre of the galaxy."

Such will be the ultimate fate of the quasar trio, says Djorgovski. Some 100 million years from now, they will have all tossed each other out. And astronomers will have to go looking elsewhere for new record-setters.

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