Nature's "zoom lens" allowing Hubble to spot distant galaxy

WASHINGTON -- The Hubble Space Telescope has used a natural "zoom lens" it discovered in space to photograph in remarkable detail an otherwise invisible distant galaxy and to chart concentrations of the mysterious dark matter that controls the fate of the universe.

A team led by Richard Ellis of Durham University in England recently was using the Hubble's wide field camera to observe a giant cluster of galaxies known as AC114. Two six-hour exposures revealed a pair of faint objects there that appeared to be perfect mirror images of each other.

The astronomers concluded that the orbiting telescope was seeing a phenomenon known as a gravitational lens, first predicted by Albert Einstein. This occurs when the gravitational field of a massive object bends light rays from a more distant source. (To get a similar effect -- of light split into arcs and rings -- scientists suggested looking through the circular base of a glass goblet turned sideways in front of a lamp.)

In this case, the lens is AC114, some 4 billion light-years away. The far galaxy in the mirror images is as much as 10 billion light-years away. That distance, coupled with the fact that the object is only 1,000 light-years across, would normally put it far beyond the vision of the Hubble, scientists said.

The huge gravitational pull of the cluster -- which contains many massive galaxies, each with perhaps 200 billion stars -- bends, concentrates and focuses the light of the distant object into mirror images: the more powerful the gravity, the greater the effect and the more images. These images were unusually far apart, indicating that the cluster had a very dense, massive core.

Such gravitational lenses have been detected previously by ground-based astronomers, as well as by the Hubble. Previously, however, the objects appeared as vague dots. This time, "We actually resolved the structure within the two images" of the distant magnified object, said Ellis.

Scientists subsequently looked for and found a third mirror image, he said.

Ellis called this ability to discern in detail the structure of the magnified object "quite a breakthrough." The gravitational "telephoto lens" both magnified and intensified the image of the far galaxy, boosting its luminosity by a factor of about 30, and also enlarging it by three or four times.

"We're likely to see images like this very often with Hubble," he said.

The degree of detail not only enables astronomers to study this very distant galaxy, which Ellis said is probably in a violent phase of star formation. It also allows them, by studying how the intervening lens bends the light, to learn more about the concentration of dark matter that appears to make up most of the mass in the cluster.

Astronomers have estimated that at least 90 percent of the universe is made of some kind of invisible material that is not detectable by existing technology, except indirectly through its gravitational effects. Because these effects ultimately will control whether the universe continues expanding or collapses back upon itself, astronomer Daniel Weedman of Pennsylvania State University said, "This is an absolutely desperate issue in astronomy."

Ellis' detailed modeling of the AC114 lens provides an important new measurement of the amount of dark matter in the cluster, confirming previous estimates based only on the motion of the galaxies within the cluster, scientists said. All of the researchers found approximately 50 times as much dark matter as visible matter.

However, the new results show that the dark matter is twice as concentrated toward the center of the cluster as are the individual galaxies. This is contrary to what had been predicted by many dark-matter theorists, astronomers said.