Gravity at play.

Astronomers are delighted to have found 19 galaxies that appear to be bent out of shape. The distorted images are cosmic mirages, arcs or rings of light created when the gravity of a massive foreground object bends and magnifies the light from a galaxy lying behind it. Albert Einstein predicted the effect, known as gravitational lensing, in 1936, but telescopes at the time weren't powerful enough to discern it.

In the study, Adam Bolton of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his colleagues combined the power of the Hubble Space Telescope with the breadth of the Sloan Digital Sky Survey. That survey of one-fourth of the sky employs a ground-based telescope in Apache Point, N.M. Using Sloan data, the team picked out large, elliptical galaxies capable of acting as gravitational lenses. When they pointed Hubble at 28 of these lensing candidates, they found arcs and rings close to 19 of them, indicating that they were indeed distorting the images of more-distant galaxies.

Eight of the 19 lensed galaxies have had their light bent into a circle called an Einstein ring. This pattern arises when one galaxy is almost exactly aligned behind another, as seen from Earth. Astronomers had previously identified only three Einstein rings.

In addition to providing curious shapes, gravitational lensing is a powerful probe of dark matter, the invisible, exotic material that theorists say resides in massive halos around every elliptical galaxy. Although dark matter halos can't be directly seen, astronomers can deduce the presence of this material by the extent to which its mass bends the light of background galaxies.

Bolton and his colleagues describe their study in the February Astrophysical Journal.--R.C.