Hubble Finds Double Einstein Ring
ScienceDaily (Jan. 10, 2008) —
The NASA/ESA Hubble Space Telescope has revealed a
never-before-seen optical alignment in space: a pair of glowing
rings, one nestled inside the other like a bull's-eye pattern.
The double-ring pattern is caused by the complex bending of
light from two distant galaxies strung directly behind a
foreground massive galaxy, like three beads on a string.
This is an image of gravitational lens system SDSSJ0946+1006 as
photographed by Hubble Space Telescope's Advanced Camera for
Surveys. The gravitational field of an elliptical galaxy warps
the light of two galaxies exactly behind it. The massive
foreground galaxy is almost perfectly aligned in the sky with
two background galaxies at different distances. (Credit: NASA,
ESA, R. Gavazzi and T. Treu (University of California, Santa
Barbara), and the SLACS team)
More than just a novelty, a very rare phenomenon found with the
Hubble Space Telescope can offer insight into dark matter, dark
energy, the nature of distant galaxies, and even the curvature
of the Universe. A double Einstein ring has been found by an
international team of astronomers led by Raphael Gavazzi and
Tommaso Treu of the University of California, Santa Barbara. The
discovery is part of the ongoing Sloan Lens Advanced Camera for
Surveys (SLACS) program. They are reporting their results at the
211th meeting of the American Astronomical Society in Austin,
Texas, USA. A paper has been submitted to The Astrophysical
The phenomenon, called gravitational lensing, occurs when a
massive galaxy in the foreground bends the light rays from a
distant galaxy behind it, in much the same way as a magnifying
glass would. When both galaxies are exactly lined up, the light
forms a circle, called an “Einstein ring”, around the foreground
galaxy. If another more distant galaxy lies precisely on the
same sightline, a second, larger ring will appear.
The odds of seeing such a special alignment are so small that
Tommaso says that they “hit the jackpot” with this discovery.
“Such stunning cosmic coincidences reveal so much about nature.
Dark matter is not hidden to lensing,” added Leonidas Moustakas
of the Jet Propulsion Laboratory in Pasadena, California, USA.
“The elegance of this lens is trumped only by the secrets of
nature that it reveals.”
The massive foreground galaxy is almost perfectly aligned in the
sky with two background galaxies at different distances. The
foreground galaxy is 3 billion light-years away. The inner ring
and outer ring are comprised of multiple images of two galaxies
at a distance of 6 billion and approximately 11 billion
SLACS team member Adam Bolton of the University of Hawaii's
Institute for Astronomy in Honolulu first identified the lens in
the Sloan Digital Sky Survey (SDSS). “The original signature
that led us to this discovery was a mere 500 photons (particles
of light) hidden among 500,000 other photons in the SDSS
spectrum of the foreground galaxy,” commented Bolton.
“The twin rings were clearly visible in the Hubble image”, added
Tommaso. “When I first saw it I said ‘wow, this is insane!’ I
could not believe it!”
The distribution of dark matter in the foreground galaxies that
is warping space to create the gravitational lens can be
precisely mapped. In addition, the geometry of the two Einstein
rings allowed the team to measure the mass of the middle galaxy
precisely to be a value of 1 billion solar masses. The team
reports that this is the first measurement of the mass of a
dwarf galaxy at cosmological distance (redshift of z=0.6).
A sample of several dozen double rings such as this one would
offer a purely independent measure. The comparative radius of
the rings could also be used to provide an independent measure
of the curvature of space by gravity.
Observations of the cosmic microwave background (a relic from
the Big Bang) favour flat geometry. A sample of 50 suitable
double Einstein rings would be sufficient to measure the dark
matter content of the Universe and the equation of state of the
dark energy (a measure of its pressure) to 10 percent precision.
Other double Einstein rings could be found with wide-field space
telescope sky surveys that are being proposed for the Joint Dark
Energy Mission (JDEM) and recently recommended by the National
The above story is reprinted from
materials provided byESA/Hubble
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