Paula Cleggett-Haleim
NASA Headquarters, Washington, D.C.                           January 13, 1992
(Phone:  202/453-1547)                                        12:30 P.M. EST

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

Dr. John Bahcall
Institute for Advanced Study, Princeton, N.J.
(Phone:  609-734-8054)

RELEASE:  92-3

NASA'S HST SNAPSHOTS PROBE THE EARLY UNIVERSE

	Astronomers are reporting surprising and interesting initial results
from a survey of several hundred quasars now being carried out with NASA's
Hubble Space Telescope (HST).  Using HST's extremely high resolution
images, this "Snapshot Survey" program has sought to detect evidence on
gravitational lensing at a level of detail not obtainable with ground-based
telescopes.

	"The HST images provide unprecedented information about the
frequency of gravitational lensing by galaxies," says Dr. John Bahcall of the
Institute for Advanced Study in Princeton, N.J.  He adds, "These results set
new constraints that cosmological models must satisfy and already rule out
some of the extreme theories."

	Drs. Bahcall, Dan Maoz, Donald Schneider and Brian Yanny, all of the
Institute for Advanced Study; Rodger Doxsey, Space Telescope Science
Institute (St ScI); Neta Bahcall, Princeton University Observatory; and Ofer
Lahav, Institute for Astronomy, Cambridge, England, are announcing their
results today to a press conference at the 179th meeting of the American
Astronomical Society in Atlanta.

	The findings set new limits on the nature and distribution of material
in the universe.  The results provide information on objects such as stars
and galaxies that can be detected with telescopes as well as non-luminous
material or "dark matter".

	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, its
existence has been inferred from its gravitational influence on the motions
of stars in galaxies and the motions of galaxies in clusters.  The phenomenon
of gravitational lensing provides a powerful probe in the search for dark
matter.

	Albert Einstein was first to point out that light, like matter, should
be deflected by gravitational fields.  If a massive object, such as a galaxy,
lies along the line of sight to a more distant object, the galaxy will act as a
gravitational lens -- the light from the background source will be amplified,
and the observed position of the background source will be slightly different
from its true location because the light rays passing near the galaxy have
been deflected.

	If the galaxy is sufficiently massive, it can bend the light rays
enough to form several images of the distant object.  By carefully analyzing
how a quasar's image is distorted, astronomers can gain insights into the
properties of the intervening galaxy and in favorable cases, derive some
information about the large scale structure of the universe.

	During the past 13 years, about a dozen examples of probable
gravitational lenses have been discovered by ground-based telescopes.
According to conventional theoretical models, approximately half of all
multiple-image gravitational lenses cannot be identified from ground-based
telescopes because the separation between the images will be less than one
arc second, which is below the nominal resolution limit for ground-based
telescopes.  Hubble Space Telescope routinely provides 0.1 arc second
resolution of moderately bright point sources, making it a unique and
powerful tool for searching for multiple-image lenses.

	The class of objects most likely to be multiply-imaged by galaxies are
distant, bright quasars.  The survey team prepared a list of 354 such quasars,
and they began collecting data in the summer of 1990.  The program was
dubbed the "Snapshot Survey" by STScI Deputy Director Peter Stockman,
because it uses HST's Planetary Camera to take short, 2-to-4-minute
exposures.  Doxsey stresses that "the snapshots are taken when there are
gaps in HST's normal science observing program, times when the telescope
would otherwise be idle."

	To use the limited time available most effectively in the scheduling
gaps, the pictures are taken using only the spacecraft gyroscopes for
pointing control.  This eliminates the need to acquire guide stars, saving 15
precious minutes per object.

	The resulting images are slightly trailed (due to spacecraft drift) yet
scientifically valuable.  "More HST data has been studied in this survey than
any other HST science program," says Bahcall.

	The Snapshot Survey was initiated on a trial basis with the support of
the STScI Director Dr. Riccardo Giacconi, who made observing time
available out of his Directors' Discretionary Time.  The mission planners at
STScI developed the techniques necessary to schedule these observations
without affecting the primary science projects.  The success of this survey
has led to the adoption of this technique for other scientific programs,
several of which were proposed for future HST observations.

	The one Snapshot Survey gravitational lens candidate, 1208+101, may
prove to be a very valuable scientific find.  The picture of 1208+101,
obtained in July 1991, shows that the quasar appears to be split into two and
possibly three images.  This result is particularly interesting because, says
Schneider, "Not only does this quasar have a very large redshift, but the
image separations are all less than half an arc second."

	Many of the Snapshot Survey quasars have been studied previously
with ground-based telescopes.  These observations indicated that several
quasars might contain multiple images separated by one arc second or less.
To date only one such previously published candidate image has been
detected in the Snapshot Survey; the HST picture does not show any
evidence for a second image.  "Of the approximately 300 quasars analyzed,
only one object appears likely to be a gravitational lens," says Maoz.

	On Dec. 23, another Planetary Camera picture of the candidate was
taken, this time with precise pointing and guiding.  The new data confirm
that the quasar consists of at least two images.

	Scientists do not know at this time if this is a case of gravitational
lensing or whether the faint image is simply a star in our galaxy that just
happens to be projected along the line of sight to the quasar.  If the fainter
image is that of the quasar, then the astronomers have made an important
discovery, for this would be the most distant known example of gravitational
lensing and thus, an excellent probe of the matter between Earth and the
quasar.  A description of the Snapshot observation of 1208+101 will appear
in the February issue of The Astrophysical Journal Letters.

	Bahcall says that scheduled HST observations of 1208+101 should
settle the issue, and the astronomers are eagerly awaiting the outcome.
Bahcall explains that in science, truth ultimately depends upon nature, not
people.  We don't know if this is an important discovery or just a chance
coincidence.  Only observations next week will tell.''

	Snapshot Survey observations continue to be taken, and the results
will take months more to analyze.  The HST data may ultimately provide new
and unexpected insights into the early universe.  Bahcall cautions, however,
that the Snapshot Survey will not offer insights into competing Big Bang
models.  "The results tell us about the denizens of the universe but not how
they got there."