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Universal spectacle

To the delight of astronomers, galaxies that bend light are popping up all over

Friday, May 14, 1999

By Byron Spice, Science Editor, Post-Gazette

It's been 20 years since astronomers found their first gravitational lens, visible proof that massive objects can warp space and bend light.

But recent findings suggest these natural magnifying glasses may soon become more than curiosities, providing a new scientific window on the universe.

Astrophysicists at Carnegie Mellon University say an ongoing sky survey with the Hubble Space Telescope has turned up at least 10 new gravitational lens candidates in a patch of sky no bigger than the full moon.

In the previous two decades, astronomers had found about 50 gravitational lenses and only a fraction of those have been usable for research.

The new findings, published in this month's issue of the Astronomical Journal, show prospects are good that hundreds might be discovered.

"There's a lot more sky if one has the time and technology to survey it," said Kavan Ratnatunga, a senior research scientist at Carnegie Mellon. "You're not limited by how many you can find, but by the time you have to find them."

Their relative abundance, if confirmed by further observations, by itself may be proof that the universe will never stop expanding.

Amassing a catalog of hundreds of gravitational lenses also would provide information about galactic evolution and perhaps other phenomena, said Joseph Lehar of the Harvard-Smithsonian Center for Astrophysics.

"It will open up opportunities we haven't even imagined," Lehar predicted.

Gravitational lenses occur when a massive object, such as a galaxy or a black hole, is aligned with a far more distant object, such as a quasar. The object in the foreground acts like a lens, its mass bending the light from the object behind it.

"They have to line up practically exactly," noted Ratnatunga, which makes gravitational lenses fairly rare. In 1936, physicist Albert Einstein, whose general theory of relativity predicts this phenomenon, was skeptical that telescopes of that time could ever observe such an event.

The effect as seen from Earth can be to split the light from the distant quasar, producing what appears to be two copies, one on each side of the lensing galaxy. If the foreground galaxy is elliptical rather than spherical, four copies may appear, forming what is called an Einstein cross.

If the lensing galaxy is precisely aligned with the quasar, the result can appear as bright arcs, or even a continuous ring of light, called an Einstein ring.

The new candidate lenses were discovered by Ratnatunga and Carnegie Mellon astrophysicist Richard Griffiths as part of a Hubble project called the Medium Deep Survey.

The survey, directed by Griffiths, is piggybacked on other observations by the Hubble. While other astronomers use the Hubble's spectrograph or other instruments to study some object of interest, the survey team opens the Hubble's wide-field camera to take a picture of whatever patch of sky is visible at the time.

So far, they have snatched long glimpses at more than 500 such patches.

Though Griffiths and Ratnatunga have picked out their Top 10 lens candidates, further observations are needed to complete the study. For instance, astronomers need to determine each object's red shift -- a measure of its distance from Earth.

This spectrographic study is relatively easy to do for the lensing galaxy, but the red shift of distant quasars out near the edge of the visible universe usually requires work on one of the world's largest telescopes, such as the Keck telescopes in Hawaii.

Because gravitational lenses tend to involve very distant objects, a relative abundance of lenses suggests that there are lots of distant objects and that the universe is fairly big with widely dispersed matter, Ratnatunga said. That, in turn, is consistent with other observations of the past two years that suggest the universe, which has been expanding since the Big Bang, will never reverse itself and end with a Big Crunch.

Earlier searches for gravitational lenses using radiotelescopes suggest that about one out of every 500 distant objects is likely to be gravitationally lensed, said Lehar, who is part of a joint Harvard-Smithsonian and University of Arizona project called CASTLeS that is surveying all known gravitational lenses.

By that rule of thumb, "I would say [the Carnegie Mellon scientists] are actually finding about as many as we would expect," Lehar said, if a little on the low side. "They clearly picked out the best ones."

In addition to predicting the fate of the universe, the study of gravitational lenses also could provide a means for estimating the mass of the lensing galaxies. Comparing mass with the galaxy's brightness might provide hints about the rate of star formation, Lehar said. But that will be possible only if astronomers can catalog hundreds of lenses, a prospect that now appears possible.



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