The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/95_43.txt

Text File | 1996-01-12 | 7KB | 143 lines

FOR RELEASE: October 18, 1995

CONTACT:  Ray Villard
          Space Telescope Science Institute, Baltimore, MD
          (Phone: [phone])

          Kavan Ratnatunga
          Johns Hopkins University, Baltimore, MD
          (Phone: [phone])

PRESS RELEASE NO.: STScI-PR95-43


         HUBBLE DISCOVERS NEW CLASS OF GRAVITATIONAL LENS
               FOR PROBING STRUCTURE OF THE COSMOS

NASA's Hubble Space Telescope has discovered a new distant class of
quadruple, or cross-shaped, gravitational lenses which might eventually
provide astronomers with a powerful new "magnifying glass" for probing
a variety of characteristics of the universe:  the distribution of dark
matter, abundance of supermassive black holes, and even determining if
the universe will expand forever or eventually collapse.

The two gravitational lenses were discovered in about 100 fields of sky
imaged by Hubble's Wide Field Planetary Camera 2 (WFPC 2).  Because the
combined area surveyed is about that of a full Moon, astronomers expect
that there may be as many as half a million similar lenses scattered
across the heavens -- though Hubble is expected to only detect about
three per year through snapshots of the sky.

Hubble's high resolution allows astronomers to extend the search to
much fainter, and hence much farther lenses, than those few examples
ground-based telescopes have uncovered relatively nearby.  Hubble can
explore a larger volume of space which could provide enough examples of
this rare cross type of lensing to allow astronomers to address a
variety of fundamental cosmological questions.

"This is a big jump for the young field of gravitational lenses --
which was theory until less than two decades ago.  This opens up a new
class of lens, which is a galaxy with well understood properties," said
Kavan Ratnatunga of Johns Hopkins University, Baltimore, MD.  "The
distinctive cross-like pattern around an elliptical galaxy makes them
unambiguous quadruple lens candidates, even before spectroscopic
observations, which are typically used to confirm lenses."

The discovery is reported by Ratnatunga and other astronomers of Johns
Hopkins University in the November 1 issue of the Astrophysical Journal
Letters. The first cross-shaped lens was discovered serendipitously by
Eric Ostrander while processing HST images for the Medium-Deep Survey,
a Hubble key project led by Richard Griffiths.  A second fainter and
smaller lens was identified a few weeks later by Myungshin Im.  Each
configuration is in the form of four faint blue images situated
symmetrically around a much brighter red elliptical galaxy.

A gravitational lens is produced by the enormous gravitational field of
a massive object which bends light to magnify, brighten and distort the
image of a more distant object.  Depending on the alignment between the
objects and the mass distribution of the foreground lens, the more
distant object can be smeared into arcs or split into pairs, triples,
or even quadruple images.

Gravitational lenses were predicted by Albert Einstein, though the
resolution of ground-based optical telescopes at the time made him
remark: "there is no great chance of observing this phenomenon".

Since 1979, few examples of lensing have been observed.  Arc-shaped,
objects are the most common, followed by pairs of lensed objects.
However, it is impossible to identify the true gravitational lenses
without observations which show the two objects have exactly the same
spectral fingerprint and so are "multiple" images of a single object.

Bright quadruple lenses, which have a distinctive cross pattern, are
extremely rare.  Only two examples are known by ground-based surveys of
the whole sky.  The lensing objects for these cases are also unusual -
the first lens, discovered in 1985 and dubbed the "Einstein cross", is
a quasar lensed by a supermassive black hole at the center of a nearby
bright galaxy; the other, discovered in 1988 and dubbed the "clover
leaf", is a quasar lensed by an unseen mass.

More than simply showing lenses as astronomical curiosities, the new
Hubble observation offers a new tool for probing the cosmos.  The
researchers say this may be as significant as the discovery of Cepheid
variable stars earlier this century, that allow astronomers to measure
cosmic distances to neighboring galaxies.

Astronomers can use quadruple lenses to estimate the density of matter
in space by comparing the redshift of the lensing galaxy and lensed
object with the geometric distance (as measured by the angular
deflection by the lens).  If the density of matter is high, then space
is positively curved and the universe will eventually collapse and come
to an end.  A low mass-density means the universe will expand forever.

"Preliminary analysis of the two new HST lenses seems to suggest that
the density of matter in space is probably low enough to allow the
universe to expand forever," said Myungshin Im.  A sample of at least a
dozen lenses of this type is needed before a more definitive estimate
can be made.

Detailed image analysis of the lens has also been used to probe the
distribution of dark matter in the foreground galaxy.  "The model for
the observed lens configuration clearly shows that the mass of these
galaxies consists of predominantly dark matter in a very elliptical
distribution", said Ratnatunga.  This shows that the mysterious dark
matter - invisible matter of an unknown type -- is more than 90 percent
of the mass of a typical elliptical galaxy and provides an upper limit
to the masses of black holes that may dwell at the centers of these
galaxies.

Hubble's Medium-Deep Survey is conducted in "parallel" mode using
Hubble's WFPC 2.  These pictures survey previously unobserved random
regions of sky while a predetermined celestial target is being observed
with another HST instrument.  The survey is being used to continue an
ongoing systematic search to find more gravitational lens candidates.

                 * * * *

The Space Telescope Science Institute is operated by the Association of
Universities for Research in Astronomy, Inc. (AURA) 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 (ESA).


Image files in GIF and JPEG format may be accessed on Internet via
anonymous ftp from ftp.stsci.edu in /pubinfo:

                                      GIF                       JPEG
PRC95-43   Gravitational Lenses    gif/MDSgrlns.gif    jpeg/MDSgrlns.jpg

The same images are available via World Wide Web from URL
http://www.stsci.edu/pubinfo/Latest.html, or via links in
http://www.stsci.edu/pubinfo/Pictures.html.


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