Summer 1997
An Illustrated Recipe for Gravitational Lenses
by Dr. Donna Womble
(The study of gravitational lenses is becoming an increasingly large part of MIRA’s research. We turned to MIRA’s Donna Womble to explain these amazing objects—Ed.)
A gravitational lens is an extraordinary astronomical object which is really made up of two separate objects. The necessary parts of a gravitational lens are (1) a luminous object called the SOURCE, and (2) a massive object called the LENS. In order to form a gravitational lens, the SOURCE must be located farther away, yet closely aligned with the LENS.
When light from the background source passes by the foreground lens, it will be deflected and sometimes magnified. These effects result in an object which appears to be a different shape or brighter than it would ordinarily appear. This combination of two objects, one behind the other, is called a "gravitational lens".
The first gravitational lens ever discovered is called the "Twin
Quasar" since its image (left-most panel) simply looks like two identical objects; it
is officially named 0957+561 A & B. The northern (upper) image is labeled
"A", while the southern (lower) image is called component "B". This
gravitational lens was discovered accidentally by Dennis Walsh, Bob Carswell, and Ray
Weymann using the Kitt Peak 2.1-m telescope in 1979 -- some 60 years after Einstein
predicted this phenomenon! Later, Alan Stockton obtained some excellent digital images of
this system using the University of Hawaii 2.2-m telescope. With these, he was able to
subtract off the quasar light from image "B" in order to 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; I drew the right panel)
The bending or magnification of the light from the source is caused by the gravitational pull of the foreground lens. It is similar to that which occurs when light passes through an ordinary glass lens -- but, with a massive body rather than a piece of glass causing the deflection. The gravitational lensing phenomenon was predicted by Albert Einstein in 1915; it is a direct result of his general theory of relativity. It was confirmed by the British astronomer Sir Arthur Eddington in 1919, when light from a distant star was seen to be bent by the Sun during a total eclipse.
The "Einstein Cross" is another excellent example of a gravitational
lens, which in this case, is quadruply-imaged! This object was also discovered
accidentally by John Huchra and colleagues in 1985 during the course of a redshift survey
of galaxies. In this case, the lensing mass is a relatively nearby spiral galaxy; 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 made available by a NASA/HST press-release over the Internet; the lower panel is
from the Digitized Sky Survey CD-ROM’s obtained via the NASA/ GSFC Skyview Internet
tool).
Like people, gravitational lenses come in several different shapes and sizes depending on what types of objects are involved, their distances from us and each other, and how close the light from the source passes by the lens. The best known type of lens occurs when light from a distant quasar (source) is deflected by close passage to a single galaxy (lens). In this case, the result appears to be several identical quasars located very close together; these usually occur as double, triple, and quadruple images of the same source. In addition to these multiply imaged objects, gravitational lenses can also appear as arcs and sometimes even complete rings of light! Arcs and rings are generally made when light from very distant galaxies passes through massive clusters of galaxies (arcs), or when a quasar is precisely aligned with the center of a single galaxy (rings).
Temporary gravitational lensing events occur when an unidentified massive object in our Galaxy passes in front of a distant star. In these instances, the star appears to brighten rapidly and then return to its normal state in a predictable manner. However, these transient events are not normally considered to be "gravitational lenses" but are called "microlensing events".
Many small gravitational arcs surround the center of the foreground cluster of galaxies called Abell 2218 as seen in this Hubble Space Telescope image. The galaxies in this cluster appear as the large, irregularly-shaped objects and together, they form the lensing mass. The arc-shaped objects appear to lie along concentric circles which are centered on the largest cluster galaxy. These arcs are the gravitationally-lensed images of much more distant galaxies. (Image also courtesy of a NASA/HST Press release obtained via the Internet).
Besides their sheer uniqueness, these unusual objects have several practical uses for the study of cosmology. Because their shape and size depend so critically upon the geometrical configuration of the source and lens, many interesting properties of the system can be determined. These properties include the measurement of the masses of the lenses, the physical properties of the very faint distant sources whose light has been magnified by lensing, the chemical composition of the intervening lens, and the absolute distances to these objects. This last topic has long been considered to be of utmost importance to astronomy because by knowing the true distance to these objects, we can determine the expansion rate of the universe - the famous "Hubble Constant"!
Gravitational lenses have become a popular research topic at MIRA. Dr. Whitney Shane is presently studying the "Einstein Cross" gravitational lens in order to use it to determine the Hubble Constant. I am involved in a collaborative effort to search for new multiply-imaged lenses which could also be used for determining this very important value. To learn more about gravitational lensing, please see the articles published in "Sky and Telescope": November 1983 page 390, April 1991 page 375, and January 1994 page 20.
This object, known as PKS 1830-211, is a great example of an "Einstein
Ring". Here, the light from a distant quasar is lensed to form two bright images
along the perimeter of a full ring of light. A complete ring is formed when light from a
distant source lies precisely behind the center of a massive object. However in this
image, this lensing mass is not apparent. This image was taken in the radio-regime using
the MERLIN Interferometer by T. Muxlow (Obtained via the Internet).