Through the Looking Glass

Through the looking glass

NASA's Hubble Space Telescope has uncovered exotic rings, arcs and crosses that are optical mirages produced by gigantic gravitational lenses in deep space

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."

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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. Web Links

STScI press release - May 13, 1999

Hubble home page - at STScI

Gravitational Lensing Tutorial - from the Goddard Space Flight Center

Ring Around a Galaxy - Astronomy Picture of the Day, Dec. 23, 1998

Hubble Deep Field South - Astronomy Picture of the Day, Dec. 14, 1998

A Bulls-eye Einstein Ring - Astronomy Picture of the Day, Mar. 30, 1998

A Galaxy Cluster Lens - Astronomy Picture of the Day, Jan. 11, 1998

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Chandra will target the age of the Universe -- Astronomers plan X-ray measurements of galaxy clusters for a new measurement of the Hubble Constant Apr. 23, 1999 NASA Science News

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