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Twisted. Distorted images of distant galaxies, like the "wheel rim" effects simulated here (above), helped astronomers pinpoint the concentrated mass of a galactic cluster (below).

Seeing Through Gravity's Lens

Astronomers are taking new strides in studying dark matter, the mysterious, unseen stuff that is thought to make up 90% of the mass of the universe. In a paper to be published in the 20 August issue of Astrophysical Journal Letters, a team demonstrates the viability of a hot, new technique that could point to dark matter by revealing the way its gravity bends light from galaxies beyond.

Several teams are racing to make use of dark matter's ability to distort passing light, a phenomenon called gravitational lensing (Science, 17 March 2000, p. 1899). Tony Tyson of Lucent Technologies' Bell Labs in Murray Hill, New Jersey, used a wide-angle camera attached to the 4-meter Blanco telescope near La Serena, Chile, to study a patch of sky about twice as big as the full moon, containing tens of thousands of galaxies but no previously known galaxy clusters.

They analyzed each galactic speck for telltale distortions that might be caused by massive but invisible objects closer to the telescope. If the objects are hefty enough, their gravitational pull bends light traveling toward Earth so that images of distant galaxies appear to wrap around the objects like the rim of a wheel (see figure). By studying the pattern of distortion, the researchers created a "mass map" of the space between Earth and the distant galaxies. In one corner of the map, they found a dense patch that indicated an undiscovered cluster of galaxies, which the team subsequently confirmed using a conventional telescope.

Next, Tyson's team set out to learn how far away the newfound lensing cluster was. They started by estimating the distance to the far-off light source galaxies by comparing their colors to those of galaxies at known distances. The farther a source lies from a gravitational lens, the more the lens bends its light. By studying how much the lensing cluster distorted the light of thousands of sources, the astronomers calculated that the cluster lay about 3.5 billion light-years from Earth. Double checking the spectra from some galaxies in the lens, the team confirmed the value to within 10%. "I am still surprised at how well it works," says Tyson's colleague David Wittman.

The new work is "the first convincing demonstration" that gravitational lensing will help astronomers map dark matter, says University of Chicago astrophysicist Wayne Hu. "I expect great things."

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