Gravity's ring: Hubble bags another lens.

When it comes to gravity, some mirages are more nearly perfect than others.

In the 1930s, Albert Einstein predicted that a massive object can act as a lens, intensifying and bending light from a body that lies behind it. The lens typically generates multiple copies of a background body or stretches its image into an arc. In rare instances, when Earth, the lens, and the distant body are exactly aligned, the distorted image takes the shape of a complete circle.

Several of these circles, known as Einstein rings, have been found since 1987. Researchers say that an image unveiled this week is the first to capture an Einstein ring, as well as the galaxy responsible for this cosmic illusion, in a single visible-light or near-infrared image.

Peter N. Wilkinson of the University of Manchester in England and his colleagues reported the find at the National Astronomy Meeting in Saint Andrews, Scotland, and in the April 1 Monthly Notices of the Royal Astronomical Society.

To minimize the blurring caused by Earth's atmosphere, many astronomers use radio telescopes to search for gravitational lensing. In their ongoing study, Wilkinson and his colleagues rely on several instruments, including the Very Large Array near Socorro, N.M., and the MERLIN network of radio telescopes spread across England, to examine thousands of distant galaxies.

A MERLIN image of a radio-emitting galaxy taken several years ago showed a partial ring, the apparent handiwork of a gravitational lens. Follow-up observations with NICMOS, the near-infrared camera on the Hubble Space Telescope, revealed a full ring as well as the lensing galaxy--a system known as B1938+666. The radio image depicted only a partial ring, notes Wilkinson, because the radio-emitting sources are not aligned precisely with the lensing galaxy.

"The scientifically interesting point is the important role NICMOS is playing... in detecting rings and partial rings," says Christopher S. Kochanek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The infrared camera is well suited to hunting lensed systems, he explains, because cosmic expansion shifts the visible light emitted by distant galaxies to infrared wavelengths.

Counting the lenses may be the best way to investigate the cosmological constant, an antigravity term introduced but later abandoned by Einstein. The term has since been resurrected to explain recent observations (SN: 3/21/98, p. 185). If the universe does have a cosmological constant, its distribution of galaxies would allow for a greater frequency of lensing.