SEATTLE—Astronomers at the California Institute of Technology and Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland are announcing today the discovery of the first known triplet of quasars. The discovery of the trio, about 10.5 billion light-years from Earth, is based in part on observations at the W.M. Keck Observatory on Mauna Kea, Hawaii.
Quasars are extraordinary luminous objects in the distant universe, believed to be powered by supermassive black holes in the cores of galaxies. A single quasar could be a thousand times brighter than an entire galaxy of a hundred billion stars, and yet this remarkable amount of energy originates from a volume smaller than our solar system. About a hundred thousand quasars have been found to date, and among them several tens of close pairs, but this is the first known case of a close triple quasar system.
"Quasars are extremely rare objects," says S. George Djorgovski, a professor of astronomy at Caltech and the leader of the team announcing the discovery at the winter meeting of the American Astronomical Society. "To find two of them so close together is very unlikely if they were randomly distributed in space. To find three is unprecedented."
The distant quasar LBQS 1429-008 was first discovered in 1989 by an international team of astronomers led by Paul Hewett of the Institute of Astronomy in Cambridge, England. Hewett and his collaborators found a fainter companion to their quasar, and proposed that it was a case of gravitational lensing.
According to Einstein's general theory of relativity, if a large mass (such as a big galaxy or a cluster of galaxies) is placed along the line of sight to a distant quasar, the light path will split, and an observer on Earth will see two or more close images of the quasar.
The first such gravitational lens was discovered in 1979, and hundreds of cases are now known. However, several groups over the past several years cast doubts that this system is a gravitational lens, and proposed instead that it is a close physical pair of quasars.
What the Caltech-Swiss team has found is that there is a third, even fainter quasar image associated with the previously known two. The discovery was made using the W. M. Keck Observatory's 10-meter telescope atop Mauna Kea, Hawaii. These data were combined with measurements obtained at the European Southern Observatory's 8.2-meter Very Large Telescope, at Cerro Paranal, in Chile.
The astronomers performed an extensive theoretical modeling, trying to explain the observed geometry of the three images as a consequence of gravitational lensing. "We just could not reproduce the data," says Frédéric Courbin of EPFL, the second author on the paper. "It is essentially impossible to account for what we see using reasonable gravitational lensing models."
Moreover, there is no trace of a possible lensing galaxy, which would be needed if the system was a gravitational lens. The team has also documented small, but significant differences in the properties of the three quasars. These are much easier to understand if the three quasars are physically distinct objects, rather than gravitational lensing mirages. Combining all these pieces of evidence effectively eliminated lensing as a possible explanation.
"We were left with an even more exciting possibility that this is an actual triple quasar," says Professor Georges Meylan, the leader of the Swiss part of the team at EPFL. Gravitational lensing can be used to probe the distribution of dark and visible mass in the universe, but quasar pairs-and now a triplet-provide astronomers with a different kind of insight.
"Quasars are believed to be powered by gas falling into supermassive black holes," says Djorgovski. "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."
If galaxy interactions were responsible for the quasar activity, having two quasars close together would be much more likely than if they were randomly distributed in space. This may explain the unusual abundance of binary quasars, which have been reported by several groups. "In this case, we are lucky to catch a rare situation where quasars are ignited in three interacting galaxies," says Ashish Mahabal, one of the Caltech scientists involved in the study.
Discoveries of more such systems in the future may help astronomers understand better the fundamental relationship between the formation and evolution of galaxies, and the super-massive black holes in their cores, now believed to be common in most large galaxies, our own Milky Way included.
This work is described in a paper submitted to the Astrophysical Journal Letters. The other authors are postdoctoral scholars Dominique Sluse of EPFL, Eilat Glikman of Caltech, and David Thompson of the University of Arizona's Large Binocular Telescope Observatory.
Images of the quasar triplet are available at: http://www.astro.caltech.edu/~george/qqq/
