File:AC114 hs-1992-25-a-full.jpg

English: NASA's Hubble Space Telescope (HST) has photographed a striking mirror-image of a very distant galaxy. The observations might unlock the secrets of the dark matter mystery which has puzzled astronomers for decades.

The image is seen through a huge cluster of foreground galaxies located four billion light-years away. The gravity of the galaxy cluster acts as a natural "magnifying glass," bending and focusing the light of the distant galaxy into several images each of which is enlarged and made brighter than would otherwise be the case. This rare combination of Hubble's powerful telescope mirrors and the natural "telephoto lens" gives astronomers new information on the nature of distant galaxies.

By studying how the lens bends the light, investigators can also deduce the amount and location of mysterious "dark matter", thought to make up most of the cluster's mass. Astronomers estimate that at least 90 percent of the universe consists of material that does not emit any radiation that can be detected by current instrumentation. Although dark matter cannot be seen directly, the phenomenon of gravitational lensing provides a powerful probe in the search for dark matter.

"We already knew that this cluster of galaxies could act as a gravitational lens from ground-based images," says Richard Ellis of Durham University, England. "The remarkable feature of the new data is the detail with which we can study background galaxies by combining the lensing phenomenon with the excellent image quality possible with HST. The unique combination has allowed us to measure the bending power of the lens very precisely, enabling us to determine the distribution of matter in the cluster regardless of whether it emits light."

Ellis and co-researchers Dr. Warrick Couch (University of New South Wales, Australia), Dr. Ray Sharples and Ian Smail (Durham University) made the discovery when observing the cluster called AC 114 in one of the first long exposures with the spacecraft's Wide Field Camera. Two six-hour exposures revealed a striking pair of faint objects close to the center of the cluster. Each image has a faint structure attached to it. These structures show perfect mirror-symmetry, as expected if both are lensed images of the same source. The images are unusually far apart for a lensed system, implying AC114 has a dense, massive core.

"Despite their wide separation, the high degree of symmetry and identical colors of the objects are a strong indication that they are images of the same source, supporting the hypothesis that AC114 is acting as a very powerful and massive lens," Ellis explained. "We believe that we are looking at a very faint distant galaxy whose blue color may indicate energetic star formation. At first we thought we were privileged to see such a dramatic feature in the first long exposure with Hubble, but we now believe that similar highly-magnified multiple images may be observed when the spacecraft looks through the centers of other massive clusters."

A Zoom Lens in Space

Albert Einstein was the first to point out that gravitational fields deflect light as well as matter. The gravitational field of a massive object, such as a cluster of galaxies, will deflect light rays from more distant sources seen close to the cluster center. This has the effect of shifting their apparent positions and magnifying and distorting their shapes and brightnesses. The greater the cluster's mass, the greater the effect. If the cluster is dense enough it can create several images of a single distant object.

Multiple-lensed systems provide astronomers with a powerful probe to investigate the form of the gravitation field of the lens. Ellis and fellow researchers have developed numerical models based on Einstein's theory. Starting from the location and shapes of the first two images, they predict the existence and location of further images. The remarkably blue color and unusual morphology of the source has enabled them to identify a third fainter candidate image.

This, and any further images similarly located, will enable the group to refine their lens model. The goal is to make it precise enough to find the distances and properties of hundreds of very faint galaxies viewed through the cluster. These objects are far too faint for more traditional distance-measuring techniques and promise to reveal the nature of the very early universe. "Just as in elementary optics, once you know the basic properties of a lens, you can examine the images it produces and figure out how far away the sources are," Ellis explained.

The Search For Dark Matter

Clusters like AC 114 are not only very useful probes for the galaxies at the limits of the universe, their lensing properties also show how much dark matter they contain. Astronomers estimate that 90 percent of the universe may consist of material that does not emit any radiation detectable by current instrumentation. Although such dark matter cannot be seen, its existence has been inferred from its gravitational influence on the motions of galaxies in clusters.

More importantly, the amount can be measured directly via gravitational lensing. The team's model for AC 114 provides an important new measurement of the amount of dark matter in AC 114 which agrees with previous estimates based on the motions of its galaxies. It also suggests that the dark matter may be more concentrated toward the center of the cluster than the individual galaxies. The group now plans to extend this work to other clusters. "We intend to use HST's superlative image quality to search for similar lensed systems in other rich clusters," said Ellis. "Using these we hope to directly probe the changes in the structure of clusters as they evolve and grow in the universe."