"Where you've got pockets of the most dark matter, that's where clusters of galaxies form," said Andrew Marble, a doctoral candidate in astronomy at the University of Arizona in Tucson. "Wide lenses probe what the universe looked like back in time and are a way of detecting dark matter."

Marble's report on quasar QSO 2QZ J1435+0008, "The Most Widely Separated Lensed QSO," was presented as a poster session at the American Astronomical Society's meeting in Atlanta.

Astronomers are searching for ways to learn more about dark matter, the majority of matter in the universe. However, dark matter doesn't emit visible light or other forms of radiation. That makes dark matter extremely difficult to study, because almost all astronomy is based on detecting radiation of one form or another.

One way to investigate dark matter is to study lensed quasars, quasars that have a celestial object such as a galaxy directly between them and Earth. In that case astronomers refer to the celestial body as a gravitational lens, because gravity from the object bends the quasar's light, much as the lens from a magnifying glass bends and refocuses light. Lensed quasars are rare, Impey said, because only 1 in 500 quasars has a celestial object perfectly lined up between it and Earth.

An astronomer observing a lensed quasar from Earth actually sees multiple images of the quasar. So seeing two quasars quite close together indicates the possibility that rather than two quasars, the astronomer is seeing the light from one quasar being bent in a way that generates duplicate images of the quasar. The more massive the gravitational lens, the stronger the effect it has on light and the farther apart the images of the quasar.

Marble and other researchers were working on a project that examined pairs of quasars that appear to be located close together. Lance Miller had given the team a list of such quasars and pointed out that one, QSO 2QZ J1435+0008, looked as if it might be a lensed quasar, rather than an actual pair of quasars. He thought that might be the case because both quasars appeared to have the same redshift, a number analogous to the distance from Earth.

In March 2003, Marble and some of his colleagues used the 6.5-meter (21-foot) diameter UA/Smithsonian MMT atop Mount Hopkins, Ariz., to look at the quasar pairs. He said, "We didn't go to the telescope expecting to find a gravitational lens because nobody had ever seen such a wide one before."

However, when the numbers about QSO 2QZ J1435+0008, started coming in, the group got pretty excited, he said. "We thought, 'This might really be a wide lens,'" but, he added, "You never know for sure until you go back to your office and analyze the data."

The team also obtained images of the quasars with the 6.5-meter (21-foot) diameter Magellan telescope at Las Campanas Observatory in Chile.

Once the data were analyzed, it was clear that the two images had the same redshift. Other aspects of the spectra, measures of the radiation energy coming from the quasars, were almost identical.

Impey said, "Quasars are not all created equal. Any two quasars at the same redshift will not have the same spectra. If it's a lens, the spectra should be virtually identical." He added, "We believe we've discovered the widest-separation lens yet. Ours is bigger than the others. It's surprising."

The distance between the two images, which is measured in arc-seconds, tells the researchers about the mass of the intervening celestial object, the gravitational lens. The two images of QSO 2QZ J1435+0008 are 33 arc-seconds apart, almost twice as far apart as the next-biggest gravitational lens known and 20 times larger than the average gravitational lens.

Because the wide separation means the mass is greater than a galaxy, Impey said the object is most likely a cluster of galaxies.

The image from the Magellan telescope shows a galaxy right in the proper place to be the lens, he said, but that image is not detailed enough to show whether there is actually a cluster present. The final confirmation will come when Marble goes to the Magellan telescope in late January to make more measurements.

"The majority of the material in the universe is dark matter -- the amount of material that people and planets and stars are made of is dwarfed by dark matter," Impey said. "These unusual wide lensing situations tell us important things about dark matter and about the evolution and structure of the universe."

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