By Paul Rincon
Science reporter, BBC News, Seattle
Astronomers have found the first example of a triple quasar, the American Astronomical Society meeting in Seattle has heard.
One quasar is rare, three are unprecedented, said Prof Djorgovski
Quasars are powerful sources of energy, thought to be powered by supermassive black holes.
At first, researchers thought the triplet was just an illusion, caused by the splitting of light beams.
But a team using Hawaii's WM Keck Observatory has found the system really involves three black holes.
Each quasar produces massive amounts of electromagnetic energy, including visible light and radio waves.
They are powered by gas falling into a black hole at the centre of a galaxy. This happens most efficiently when galaxies collide and merge.
A single quasar could be a thousand times brighter than an entire galaxy of a hundred billion stars.
Professor George Djorgovski, from the California Institute of Technology in Pasadena, and colleagues studied a system called LBQS 1429-008.
It was found by another group of astronomers in 1989. They reported that it seemed to consist of two quasars.
At first, it was thought that one of the quasar pair was a mirage caused by an effect known as gravitational lensing.
This effect is caused by an object of large mass getting in the path of light coming from the quasar. This splits the light beams, in essence creating a double image.
But astronomers have since identified around 100,000 quasars and dozens of genuine binary quasars.
Professor Djorgovski's team found a third, faint quasar using one of the Keck's 10m telescopes and measurements from one of the European Southern Observatory's 8.2m telescopes in Chile.
The team used computer modelling to see if the observations could be easily explained by gravitational lensing. This explanation turned out not to be a good fit.
If the triple quasar had been down to lensing, the astronomers should have seen four quasar sources, not three. There would have to be something hiding one of the images.
There was no sign of a galaxy, or cluster of galaxies, that could have been the cause of the lensing effect.
The team has also documented small, but significant differences in the properties of the three quasars.
This observation is much easier to understand if the three quasars are physically distinct objects, rather than mirages.
"Quasars are extremely rare objects," says Professor Djorgovski. "To find three is unprecedented".
Professor Djorgovski thinks that the distribution of quasars in the Universe is not random.
Instead, he thinks that the collision and merging of galaxies - and the supermassive black holes that reside at their centres - may actually fuel these powerful sources of energy.
This could explain why there are more than the expected number of binary quasars.
The quasar is being seen during a period of cosmic time when such interactions between galaxies were at their height.
These phenomena may even play an important role in regulating galaxy growth, leading to the joint formation of galaxies and their supermassive black holes - which power the quasars.
Professor Djorgovski said it was also possible that quadruple quasars could yet be found.