Scientists have confirmed that two distant hyperactive galaxies are in fact a pair of similar objects known as quasars, and that no optical illusions had intervened as some researchers had feared.
The quasars are super bright galaxies powered by supermassive black holes, seen as they appeared when the universe was young. Because the two objects appear nearly identical when studied in optical wavelengths, researchers suspected they might have been the same object.
Such a trick of light would be made possible by a phenomenon called gravitational lensing, in which the gravity of an intervening galaxy cluster bends light and can sometimes create twin images of very faraway objects.
The two quasars, called Q2345+007 A and B, are some 11 billion light-years away and were found nearly two decades ago.
The new X-ray observations, made using NASA' Chandra X-ray Observatory, reveal a rare double quasar system. The objects might have been created when two galaxies merged, something scientists speculate happened a lot in the early universe.
"When galaxies interact or merge, they become more active and luminous and can excite quasar activity in their centers," said Paul Green of the Harvard-Smithsonian Center for Astrophysics, who led the research team. "The quasars that make up these nearly identical twins appear to have been hatched in the same nest."
Quasar pairs that are seen close to one another on the sky and are at the same distance from Earth often turn out to be an illusion as part of a gravitationally lensed system. In these cases, the image of a single quasar has been split into two or more images as its light has been bent and focused on its way to Earth by the gravity of an intervening massive object like a galaxy, or a cluster of galaxies.
The quasar pair studied by Chandra was thought to be such an illusion because of the remarkably similar patterns of the light, or spectra, from the pair at both optical and ultraviolet wavelengths.
But no one had found enough intervening material to produce the gravitational effects necessary to create the illusion.
The Chandra images, the most sensitive ever taken for this type of search, showed no evidence for a massive dark cluster. Further, the X-ray spectra of the two quasars were distinctly different.
"This may mean that the pair ... actually consists of two separate quasars," said Green. "However, a mystery remains. How can two quasars have identical optical spectra - every bump and wiggle? The coincidence seems improbable."
One possible explanation is that the quasars are formed close by each other grow up to look alike at optical wavelengths, but that X-rays which probe closer to their central black holes, bring out the individual differences.
Chandra observed the quasars on May 26, 2000. The results were released on March 13.
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