Scientists have unraveled a longstanding mystery about a rare double
quasar system 11 billion light years from Earth using NASA’ Chandra
X-ray Observatory. These “twin” quasars, previously thought to be an
optical illusion, were instead probably created by merging galaxies and
may have been more common in the dense Universe soon after the Big Bang.

“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, Cambridge, Mass., who led
the research team. “The quasars that make up these nearly identical
twins appear to have been hatched in the same nest.”

The Chandra data show that the quasars – luminous galaxies powered by
central supermassive black holes – are not mirror images caused by a
cosmic phenomenon known as a “gravitational lens.” Rather, these two
quasars are distinct objects that were probably spawned when their host
galaxies collided, energizing the flow of gas onto their central black

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.

Usually, the intervening mass shows up as a fainter galaxy or cluster of
galaxies seen between or among the quasar images, confirming the cause
of the illusion. The quasar pair Q2345+007 A, B 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.

However, almost two decades after its discovery by optical astronomers,
the identification of enough intervening material to “split” the image
of a single quasar into an apparent double has proved fruitless. This
led to the speculation that the gravitation light-bending might be
caused by a new type of cluster that contained hot gas and dark matter,
but failed to ever make stars or galaxies. Such a “dark cluster” would
be invisible to optical and ultraviolet telescopes, but would be
detectable in X-rays.

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 Q2345+007A,B 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 Q2345+007 on May 26, 2000, for 65,000 seconds using the
Advanced CCD Imaging Spectrometer instrument. Scientists from the
Harvard-Smithsonian Center for Astrophysics and the National Optical
Astronomy Observatory, Tucson, Ariz., were also members of the research

The ACIS camera was developed for NASA by Pennsylvania State University,
University Park, Pa., and the Massachusetts Institute of Technology,
Cambridge, Mass. NASA’s Marshall Space Flight Center in Huntsville,
Ala., manages the Chandra program. TRW, Inc., Redondo Beach, Calif., is
the prime contractor for the spacecraft. The Smithsonian’s Chandra X-ray
Center controls science and flight operations from Cambridge. The
National Optical Astronomy Observatory is operated by the Association of
Universities for Research in Astronomy (AURA), Inc., under a cooperative
agreement with the National Science Foundation.

Images and additional information about this result are available at: