From: baalke@kelvin.jpl.nasa.gov (Ron Baalke)
Subject: HST Uses Nature's Lens to Explore the Cosmos
Followup-To: sci.space
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Keywords:  HST
Reply-To: baalke@kelvin.jpl.nasa.gov
Organization: Jet Propulsion Laboratory
Date: Fri, 9 Oct 1992 02:21:49 GMT

Paula Cleggett-Haleim
Headquarters, Washington, D.C.        October 8, 1992
(Phone:  202/358-1547)

Jim Elliott
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-6256)

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
(Phone:  410/338-4514)

RELEASE:  92-168

HUBBLE USES NATURE'S LENS TO EXPLORE THE COSMOS

     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
that have puzzled astronomers for decades.  Understanding the nature of
dark matter might lead to predictions of whether the universe will expand
indefinitely or collapse of its own gravity.

     The mirror 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 lens or magnifying glass, bending, concentrating and focusing
the light of the distant galaxy into several images, each of which is bigger
and brighter than otherwise would 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," says Richard Ellis of Durham University, England.

     By studying how the natural lens bends the light, investigators also can
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 detectable 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 from ground-based images that this cluster of
galaxies could act as a gravitational lens," says Ellis.  "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 or not 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 AC114 in one of the
first long exposures with the spacecraft's Wide Field Camera.

     Two, 6-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 near-
identical colors of the objects are a strong indication that they are images
of the same source, supporting the hypothesis that we have discovered a
very massive lens," Ellis explained.

     "We believe that we are looking at a very faint, distant galaxy
undergoing an energetic period of 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 will 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 brightness.  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 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 school 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

     Although dark matter cannot be seen, its existence has been inferred
from its gravitational influence on the motions of galaxies in clusters.
Clusters like AC114 are not only very useful probes of the galaxies at the
limits of the universe, their lensing properties also show how much dark
matter they contain.

     More importantly, the amount can be measured directly via
gravitational lensing.  Ellis' model for AC114 provides an important new
measurement of the amount of dark matter in AC114 which agrees with
previous estimates based on the motions of its galaxies.

     It also suggests, however, that the dark matter is more concentrated
toward the center of the cluster than the individual galaxies.  This is
contrary to the predictions of models in which the dark matter is made up
of subatomic non-interacting particles.

     The group plans to extend this work to other clusters at different
distances.  This will allow the researchers to probe the universe at different
times in the distant past (because of the effect of light-travel time).  Such
observations will enable them to follow the evolution of the dark and visible
matter independently.

     "We intend to use HST's superlative image quality to search for similar
lensed systems in other rich clusters," said Ellis.  "Using these we will be
able to directly probe the changes in the structure of clusters as they
evolve and grow in the universe."

     The Space Science Telescope Institute is operated by the Association of
Universities for Research in Astronomy, Inc., for NASA under contract with
the Goddard Space Flight Center, Greenbelt, Md.  The Hubble Space
Telescope is a project of international cooperation between NASA and the
European Space Agency.

                                           - end -

EDITORS NOTE:  A photograph of the gravitational lens in galaxy cluster
AC114 is available to news media by calling the NASA Headquarters
Broadcast and Imaging Branch at 202/453-8375.

          Color:     92-HC-674
          B&W:     92-H-730

A videotape to accompany this release containing animation of the
gravitational lens also is available at the Broadcast and Imaging Branch.
     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | baalke@kelvin.jpl.nasa.gov
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | Einstein's brain is stored
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | in a mason jar in a lab
|_____|/  |_|/       |_____|/                     | in Wichita, Kansas.

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