Fall 1997

On 20 September, Dr. Donna Womble of MIRA told how she participated in the exciting discovery and confirmation of "A Very Special Gravitational Lens" by an international team of 20 astronomers of which she is still a member.

If you've seen the movie "Contact", you should remember the site where Jodie Foster found the intelligent signal. This is the Very Large Array (VLA) in New Mexico where Dr. Womble and her team made the initial discovery of this unique object.
 

The "Einstein Cross" gravitational lens. On the digitized Palomar Sky Survey (lower panel), it appears to be just an ordinary galaxy. However, surrounding the very center of this galaxy (labeled GC) are four identical images of a distant quasar. (The upper panel is a Hubble Space Telescope image of the Einstein Cross; the lower panel is from the Digitized Sky Survey CD-ROM).

The team study was called CLASS (Cosmic Lens All Sky Survey). Their intention was to examine as many probable lens objects as possible using the Very Large Array in New Mexico. The signals from the VLA can be combined to produce very high resolution radio images. CLASS examined 7723 objects, each for only 30 seconds, and found at least 5 new gravitational lenses so far, which is 25% of the total known to date!

The first lens candidate found (known by its exciting catalog name 1608+656!) appeared to be 1 quasar lensed to look like 4, thus the nickname of "The Quad", much nicer but less specific. Such an unusual find must be confirmed in every way possible.

Step 1: Confirm the initial observation using another array of radio telescopes. The MERLIN array of 7 dishes across England was selected, which includes the Lovell 76 meter radio telescope at Jodrell Bank. The larger separation of the array allows greater resolution than the VLA.

Result: The original observation was confirmed, with exactly the same relative positions and intensities as the VLA!

Step 2: Confirm using a different wavelength since all radiation from the source must be bent the same way by the lens. The team selected infrared measurements with the 5 meter Palomar telescope, which should show the Quad with exactly the same shape but less resolution due to the atmosphere, wavelength, and single telescope.

Result: The Quad is a gravitational lens!! The IR image a bit fuzzier than the radio maps but all 4 images are in the identical relative positions.

A review of how gravitational lenses work, using the example of the "Twin Quasar." The northern (upper) image is labeled "A", while the southern (lower) image is called component "B". When the quasar light from image "B" is subtracted, you can see the adjacent lensing galaxy, labeled "G" (middle panel). The right-most panel schematically shows how gravitational lensing in this system works. The true position of the quasar is marked "Q". In the absence of a foreground mass (galaxy), some light rays from the quasar would follow the solid lines. However, when a massive galaxy is placed at position "G", the light rays are bent (or lensed) by the galaxy's gravitational pull such that they appear to follow the dotted lines. The result is that we see two images, "A" and "B", rather than the original quasar image "Q". (The left two images are from Stockton 1980, Astrophysical Journal, V.242, p.L141; Dr. Womble drew the right panel) 

Step 3: Gather data on the nature of the source and lens objects using the powerful spectrograph with the largest telescope on earth, the 10 meter Keck on Mauna Kea, the premier optical astronomy site in the world.

At 14,000 feet, Mauna Kea ('White Mountain' in Hawaiian for the perpetual snow at the summit) involves some peculiar working conditions which Dr. Womble humorously related. Examples include the effect of diminished oxygen on one's ability to think at the telescope and the strange looks one gets on the airplane to Hawaii carrying heavy down clothes while everyone else is wearing shorts and Hawaiian shirts! It was great to hear how astronomers set up and work with the giant Keck telescopes.

Result: A very unusual spectrum was found! After careful analysis, the combined spectrum of the source and lens was determined to be an elliptical galaxy lensed by a spiral galaxy, the FIRST ever of both source and lens as galaxies!

Step 4: Examine this unique object in visible light with the Hubble Space Telescope to get high optical resolution above the earth's atmosphere.

Result: The images showed the FIRST optical ring ever! The light from the elliptical galaxy (source) has been bent into a circle of interconnecting arcs. The details of this discovery will be published in the near future.

Dr. Womble closed with a discussion of why astronomers are so interested in gravitational lenses. They are hoping that the study of these phenomena will allow the size and age of the Universe to be determined. Hubble's Law states that recession velocity (red shift) is proportional to a galaxy's distance. Currently, scale is known only to a factor of 2 -- that is, a galaxy may be 1/2 or twice as far as we think. If we can find the distance to any galaxy, the distance for all will be fixed. 

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