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Gravity's lens: Hubble gets sharpest image.

Examining a distant cluster of galaxies, the Hubble Space Telescope has produced the sharpest picture ever of a cosmic mirage called gravitational lensing. Researchers say that as Hubble takes more of these high-resolution images, they will yield a more accurate estimate of the universe's dark matter -- material that can't be observed like ordinary matter.

Richard S. Ellis of Durham University in England and his colleagues weren't searching for a gravitational lens when they began analyzing a Hubble image, taken last November, of a remote galactic cluster called AC114. But graduate student Ian Smail spotted two unusual structures on opposite sides of the Hubble picture. The objects were nearly mirror images of each other, indicating that dark matter in the cluster, which lies 4 billion light-years from Earth, acts as a giant lens. The gravity field of the unseen material had bent starlight from a more distant body, much as a distorted glass lens might create multiple images from a single light source.

This gravitational illusion, based on the principle that gravity distorts the path of light, was first proposed by Albert Einstein in 1916, and astronomers have seen the handiwork of gravitational lenses with ground-based telescopes since the late 1970s.

But in previous images, both those captured on the ground and those made by Hubble, the lensed objects appear as blurs. Ellis says that the unprecedented sharpness of the new Hubble picture, which shows L-shaped structures at the edges of the lensed images, enabled him to determine more accurately the concentration and mass of the material in AC114 that acted as a lens. His team has identified a third lensed image of the same distant galaxy in the Hubble photograph, he adds.

Assuming that the gravitational lens coincides with the center of the cluster and obeys similar laws of physics, Ellis calculates that the hidden material has 50 to 100 times the mass of visible material in the cluster and is much more densely concentrated. He reported the findings last week at a press conference in Washington, D.C.

The results match estimates of the amount of dark matter in galactic clusters made by observing lensed images from the ground. But J. Anthony Tyson of AT&T Bell Laboratories in Murray Hill, N.J., likens the impact of Hubble's single set of high-resolution lensed images to the diagnostic power of a single brain scan: One or two images may reveal only limited detail. He asserts that the blurrier but far more numerous lensed images seen from Earth still provide the best estimates of the amount of dark matter.

While Ellis disagrees, both scientists say the real revolution in the way astronomers calculate dark matter will come once Hubble begins to record many more gravitationally lensed objects.

Astronomers first postulated the existence of dark matter because the observed mass of the universe is too small to bind large objects gravitationally. The total amount of dark matter may determine the universe's fate -- whether the cosmos has the gravitational power to collapse in on itself or will expand forever.

Ellis notes that the spectra of the lensed galaxy, recorded from Earth, suggest the galaxy lies 8 to 12 billion light-years away.

In a separate report at the press conference, C. Stuart Bowyer of the University of California, Berkeley, announced that another space-borne observatory, the recently launched Extreme Ultraviolet Explorer, detected a quasar-like object about 2 billion light-years from Earth. Researchers had thought the observatory would detect few, if any, sources outside our galaxy because of the patchy fog of hydrogen that bathes the stars and absorbs extreme-ultraviolet radiation (SN: 5/23/92, p.344).
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Title Annotation:gravitational lens
Author:Cowen, Ron
Publication:Science News
Date:Oct 17, 1992
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