Press Release

Galaxy Cluster Found Using Gravitational Distortion, Suggests Independent Test of Accelerating Universe

By SpaceRef Editor
August 15, 2001
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Astronomers using the National Science Foundation’s Blanco Telescope in
Chile have used the distorting effects of a weak gravitational lens to
discover and locate a dim cluster of at least 15 galaxies at a significant
distance from Earth, using only the mass properties of the cluster, not its
visible light.

This first-time accomplishment raises clear prospects for a powerful technique
called 3-D mass tomography to conduct large-scale searches for dark matter.
This approach may provide a valuable independent check of current theories
about the accelerating expansion of the Universe.

“Most of the ‘big action’ in the Universe is governed by the mass of objects,
not by their visible light or other electromagnetic radiation,” explains
Anthony Tyson of Lucent Technologies’ Bell Labs, Murray Hill, NJ. “This
fundamentally new approach lets us measure mass instead of light, based on
simple physics that circumvents much of the unavoidable bias inherent in
trying to use tiny variations in brightness to derive critical properties of
distant objects.”

Several recent astronomical discoveries have taken advantage of the fact that
massive objects bend the path of light from more distant objects located
behind them, as seen from Earth. This property, called gravitational lensing,
was predicted by Albert Einstein.

In this case, the large mass of the newly found galaxy cluster forms a weak
intervening lens that shears, or unevenly bends, the light from more distant
galaxies behind it. The distorted, elongated images of the background
galaxies were identified and processed by a computer program. More
importantly, this program generated an estimate for the unknown cluster’s
redshift, or its velocity of recession from Earth. This redshift matched
precisely with more traditional spectroscopic redshift measurements taken
as an independent check by the research team.

A detailed research paper on the newly detected galaxy cluster will be
published in the August 20 issue of Astrophysical Journal Letters.

“We have shown that we can measure the distance to the cluster by charting
the amount of shear versus the distance to the background galaxies,” says
lead author David Wittman of Bell Labs. “This lets us locate the cluster
in its proper place in three-dimensional space, all without resorting to
studying its light. This is important because most of the mass in the
Universe is dark.”

The team used the National Science Foundation’s Blanco 4-meter telescope at
Cerro Tololo Inter-American Observatory (CTIO), outfitted with an instrument
called Big Throughput Camera, to take numerous images in four different
filters of a relatively barren portion of the visible sky, in the direction
of the constellation Pisces.

This effort yielded rough color-based (or photometric) redshifts for various
objects in the images. A computer program then sifted through the images,
looking for telltale distortions in the shape of the objects by intervening
clumps of matter. The program sorts the distortions into different “bins”
based on their locations and shapes, and then constructs a full three-
dimensional map of the region of space.

The predicted astronomical redshift of the galaxy cluster and its mass
properties match precisely with spectroscopic measurements made using the
W. M. Keck telescopes on Mauna Kea, HI, at almost exactly the predicted
redshift of z = 0.276. This number is equivalent to looking back in time
to how the cluster appeared about three billion years ago.

“With a bit more experience we will be able to do a blind search and
effectively become ‘intergalactic prospectors’ for extremely faint clumps
of mass in any direction, out to about one-half the estimated age of
the Universe,” Tyson says. “This will offer a test of the idea of an
accelerating Universe that is completely independent of the supernovae
observations which have formed the basis of this theory.”

This research result is a precursor to a larger project led by Tyson called
the Deep Lens Survey, one of more than a dozen major sky surveys supported
by the National Optical Astronomy Observatory (NOAO). Further information
about the Deep Lens Survey, which should reveal scores of such clusters
using the weak lens effect, is available on the Internet at:
http://dls.bell-labs.com/

Through Lucent Technologies’ Bell Labs, Tyson and Wittman are partners with
NOAO and the University of Arizona’s Steward Observatory in developing a
detailed concept for a future ground-based facility called the Large Synoptic
Survey Telescope (LSST). A top priority of the most recent decadal survey of
astronomy, the LSST features an 8-meter class mirror with an extraordinarily
large, three-degree field-of-view that promises to image the entire sky to
a faint stellar magnitude (R = 23.9) every few nights.

“If we can do this kind of analysis over a large-enough area, we can use
statistics to tell us how mass has clumped together over cosmic time, and
see how well the results match with the idea of accelerating expansion,”
Tyson explains. “We’ll need billions of background galaxies to do a full-up
investigation, and LSST is the only thing on the horizon that can do the
job.”

Other co-authors of the upcoming paper are Vera Margoniner of Bell Labs, Judy
Cohen of the California Institute of Technology, Pasadena, CA, and Ian
Dell’Antonio of Brown University, Providence, RI.

A camera image and a related mass map used to locate the galaxy cluster are
available on the Internet at:
http://www.noao.edu/outreach/press/pr01/pr0111.html

Located east of La Serena, Chile, CTIO is the southern hemisphere site of the
National Optical Astronomy Observatory, which is based in Tucson, AZ. NOAO is
operated by the Association of Universities for Research in Astronomy (AURA),
Inc., under a cooperative agreement with the National Science Foundation.

:: :: ::

NOTE TO REPORTERS: David Wittman can be reached at dwittman@lucent.com; Tony
Tyson can be reached at tyson@bell-labs.com .

IMAGE CAPTIONS:

[Image 1: http://www.noao.edu/outreach/press/pr01/0111images.shtml]
This image from the National Science Foundation’s 4-meter Blanco Telescope in
Chile was used to detect and locate a cluster of 15 galaxies at a significant
distance from Earth, using only the mass properties of the cluster, not its
visible light.

The image was taken with an instrument called Big Throughput Camera,
looking at a relatively barren portion of the sky in the direction of the
constellation Pisces.

The galaxy cluster is enclosed by the circle in the lower right-hand corner
of the image. The predicted astronomical redshift of the cluster and its
mass properties match precisely with spectroscopic measurements made using
the W. M. Keck telescopes on Mauna Kea, HI, at almost exactly the predicted
redshift of z = 0.276. This number is equivalent to looking back in time to
how the cluster appeared about three billion years ago.

Credit: Lucent Technologies’ Bell Labs/NOAO/AURA/NSF

[Image 2: http://www.noao.edu/outreach/press/pr01/0111images.shtml#map]
This map represents the distribution of mass in a portion of the sky in the
direction of the constellation Pisces that was imaged by the National Science
Foundation’s 4-meter Blanco Telescope in Chile.

The bright red spot in the lower right-hand corner identifies a newly
discovered cluster of 15 galaxies at a significant distance from Earth, which
was found using only the mass properties of the cluster, not its visible light.

The predicted astronomical redshift of the cluster and its mass properties
match precisely with spectroscopic measurements made using the W. M. Keck
telescopes on Mauna Kea, HI, at almost exactly the predicted redshift of
z = 0.276. This number is equivalent to looking back in time to how the
cluster appeared about three billion years ago.

Credit: Lucent Technologies’ Bell Labs/NOAO/AURA/NSF

SpaceRef staff editor.