FROM A SPACE
TELESCOPE SCIENCE INSTITUTE PRESS RELEASE
HST's Top Ten Lens
Candidates
A - HST 01248+0351 is a lensed
pair on either side of the edge-on disk lensing galaxy.
B - HST 01247+0352 is another pair of bluer lensed source
images around the red spherical elliptical lensing galaxy.
C - HST 15433+5352 is a very good lens candidate with
a bluer lensed source in the form of an extended arc about the
redder elliptical lensing galaxy.
D - HST 16302+8230 could be an "Einstein ring"
and the most intriguing lens candidate.
E - HST 14176+5226 is the first, and brightest lens system
discovered in 1995 with the Hubble telescope. The elliptical
lensing galaxy is located 7 billion light-years away, and the
lensed quasar is about 11 billion light-years distant.
F - HST 12531-2914 is the second quadruple lens candidate
discovered with Hubble.
G - HST 14164+5215 is a pair of bluish lensed images symmetrically
placed around a brighter, redder galaxy.
H - HST 16309+8230 is an edge-on disk-like galaxy (blue
arc) which has been significantly distorted by the redder lensing
elliptical galaxy.
I - HST 12368+6212 is a blue arc in the Hubble
Deep Field
J - HST 18078+4600 is a blue arc caused by the gravitational
potential of a small group of 4 galaxies. |
May 14, 1999: The NASA Hubble Space Telescope
serendipitous survey of the sky has uncovered exotic patterns,
rings, arcs and crosses that are all optical mirages produced
by a gravitational lens, nature's equivalent of having a giant
magnifying glass in space.
A gravitational lens is created when the gravity of a massive
foreground object, such as a galaxy or black hole, bends the
light coming from a far more distant galaxy directly behind it.
This focuses the light to give multiple or distorted images of
the background object as seen by the observer.
A quick look at over 500 Hubble fields of sky has uncovered 10
interesting lens candidates in the deepest 100 fields. This is
a significant increase in the number of known optical gravitational
lenses. Hubble's sensitivity and high resolution allow it to
see faint and distant lenses that cannot be detected with ground-based
telescopes whose images are blurred by Earth's atmosphere. An
analysis of this "Top Ten" list of Hubble gravitational
lenses is published by Kavan Ratnatunga and Richard Griffiths
of Carnegie Mellon University in the May issue of the Astronomical
Journal.
The amount of gravitational lensing in the universe depends strongly
on the cosmological constant, a hypothesized repulsive force
that indicates the universe is older and larger than without
this force. Therefore a large cosmological constant implies a
larger number of more distant objects whose light can, by chance,
pass close to a massive galaxy on its way to Earth and appear
lensed.
The 100 Hubble fields cover a total area equal to that of the
full Moon. Hubble's ability to see so many of these lenses in
a small fraction of the sky takes them from being a scientific
curiosity to serving as a potentially powerful tool for probing
the universe's evolution and expansion.
"In fact, these much more distant gravitational
lenses are potentially more valuable to derive fundamental cosmological
parameters than relatively closer lenses discovered from ground-based
observations," says Ratnatunga. "Follow-up spectroscopic
observations are now needed to verify that the object is far
more distant than the lensing galaxy seen at the center, as well
as to derive better distance estimates to confirm that multiple
images really belong to the same object. These are however very
difficult observations even for the largest ground-based telescopes."
The Hubble images in which these lenses
were discovered are part of the Medium Deep Survey database.
The survey catalog contains over 200,000 objects, mostly faint
galaxies. The public can search
the catalog at the Space Telescope Science Institute website
and study the myriad of never-before-seen galaxies from this
huge Hubble database on their own home computer. Users can call
up one of 500 survey fields and mouse-click on any galaxy image
to see a full resolution view of the galaxy and estimates of
its shape and brightness. Visitors can even look for patterns
that may be caused by a gravitational lens. Hubble astronomers
expect that there could be a few hundred more lenses which are
more difficult to identify confidently in these images.
Right: Albert Einstein predicted
that the gravitational field of a massive galaxy would bend light
traveling to Earth from distant quasars. This is what is called
"gravitational lensing," since the intervening galaxy
acts as a lens to focus the image of the distant quasar to a
new location. Gravitational lensing can produce multiple images,
rings, or arcs, depending on the distribution of mass in the
galaxy and the Earth-galaxy-quasar geometry.
In 1936 Albert Einstein computed the gravitational deflection
of light by massive objects and showed that an image can be highly
magnified if the observer, source and the lensing object are
well aligned. However, the lensed image separations were predicted
to be so small in angular size, Einstein knew they were beyond
the capabilities of ground-based optical telescopes. This made
him remark that "there is no great chance of observing this
phenomenon."
It wasn't for another 40 years since Einstein's conclusion that
the first gravitational lens was discovered in 1979. Several
bright and nearby lenses have been discovered since then from
ground-based observations.
Further lens discoveries required Hubble's high resolution Wide
Field Planetary Camera 2 (WFPC2) which allows the search extended
to much fainter and farther objects. It is expected that the
Advanced Camera for Surveys, to be installed on Hubble in the
year 2000, will be able to discover many more gravitational lenses
because of its sensitivity and relatively wide-angle coverage.
Right:
Almost all of the bright objects in this Hubble Space Telescope
image are galaxies in the cluster known as Abell 2218. The cluster
is so massive and so compact that its gravity bends and focuses
the light from galaxies that lie behind it. As a result, multiple
images of these background galaxies are distorted into faint
stretched out arcs - a simple lensing effect analogous to viewing
distant street lamps through a glass of wine. The Abell 2218
cluster itself is about 3 billion light-years away in the northern
constellation Draco. |