January 13, 1997
CONTACT:
Department of Physics and Astronomy David Batuski,
207-581-1036 (e-mail, [email protected])
Department of Public Affairs Nick Houtman, 207-581-3777
LONGEST SUPERCLUSTER FOUND IN AQUARIUS
TORONTO -- Today, at the American Astronomical Society meeting in
Toronto, Canada, astronomers Kurt Slinglend, David Batuski, and Chris
Miller of the University of Maine, presented evidence for what appears
to be the longest single structure yet seen in the universe, a
supercluster of galaxies about one billion light-years in length. The
discovery adds significantly to the emerging picture of large-scale
structure in the present-day universe.
The evidence also suggests that the supercluster is most certainly a
remnant of the Big Bang itself and thus provides researchers insight
into conditions in the early universe. The work was supported by the
National Science Foundation.
The supercluster consists of a string of 23 clusters of galaxies
running roughly along the line of sight from earth. They feature a
range of red shifts corresponding to distances between 1 billion and 2
billion light-years from earth in the southern part of the
constellation Aquarius. Also intriguing is a super-dense knot of seven
clusters about halfway out along the filament.
Each cluster consists of dozens of bright galaxies and probably
thousands of much fainter galaxies, all bound together by mutual
gravitation. The clusters are like beads on a string. They form a
filamentary supercluster which, in contrast to the individual clusters,
is far too large to be held together by gravity. Since the supercluster
could not have been formed by gravitational forces, it must have been
formed from perturbations in the original fireball of the Big Bang,
Batuski suggests.
In the summers of 1994 and 1995, Batuski and Slinglend made their
observations with Chantal Balkowski, who heads a team of collaborators
from Meudon Observatory in France at the European Southern Observatory
(ESO) in Chile. Batuski is an associate professor of physics and
astronomy and Slinglend a recent Ph.D.. recipient at the University of
Maine.
Using the ESO 3.6-meter (140-inch) telescope, the team collected light
spectra from several of the galaxies that make up each observed
cluster. Batuski, Slinglend, and graduate student Chris Miller analyzed
the data at the University of Maine.
Previous supercluster discoveries of similar scale include the
Perseus-Pegasus supercluster, which is also in the shape of a filament,
and the "Great Wall," which is actually a gigantic sheet of galaxies.
The well-defined portion of Perseus-Pegasus has an extent of about
two-thirds that of the Aquarius supercluster, and the largest dimension
of the Great Wall is about half as long as Aquarius. Structures of this
size match up nicely with the smaller features observed in the findings
from NASA's Cosmic Background Explorer (COBE) survey of the remnant
radiation from the Big Bang.
In 1992, NASA announced that this radiation had `wrinkles' (density
variations) that extended for distances that correspond to many
billions of light-years in today's universe.
A striking feature within the supercluster is the unusually dense knot
of seven clusters. "The knot is as tight a structure as we ever see
among clusters of galaxies, so this supercluster is an example of two
types of rare structures," said Batuski. Astronomers have found only
two other regions in the universe with clusters as densely packed as in
this knot. One, in Corona Borealis, is similar in shape and size to
the Aquarius knot but not associated with any other structure. The
other can be found in the Southern Hemisphere and has been dubbed the
Shapely Concentration, after Harlow Shapely, a famous early twentieth
century astronomer.
Batuski explains that clusters of galaxies are actually a common
feature in the night sky. In 1958 George Abell studied thousands of
photographic plates and created a catalog of 2,700 clusters of galaxies
in the Northern Hemisphere. That catalog was later expanded with Harold
Corwin and Ronald Olowin in 1989 to include 1,600 more clusters in the
Southern Hemisphere. It is from both of these catalogs that specific
clusters were chosen to be studied for this program of observations.
For the observations at ESO, a new instrument the Meudon-ESO
Fiber-Optic Spectrograph (MEFOS) was used. It collects light from up
to 29 galaxies at one time. The light from each galaxy is channeled by
an optical fiber to a grating where the light is spread out into a
spectrum before being recorded by a CCD (charge-coupled device) camera.
The observing process is still fairly time-consuming. It takes about
one hour of observing time for each cluster, but with such recent
advances in the equipment, astronomers can now obtain more data in one
night then they could in a week's time only ten years ago. With so many
astronomers competing for limited time on the world's largest
telescopes, it has become vital for scientists to become more
efficient.
The researchers at the University of Maine and Meudon Observatory hope
to study the region of the supercluster in greater detail. They will
focus on other clusters that could also be connected to the
supercluster. They will also study the space between the clusters for
"bridges" of galaxies to see just how thoroughly this supercluster is
strung together.
Eventually, Batuski says, astronomers will be able to describe the
structure on all scales in the universe with confidence. "Only so much
of it has been sampled thus far," he says. "There are plenty of fairly
bright galaxies to look at that have not been mapped. We haven't gone
very deeply into the universe yet."
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A diagram can be obtained over the Internet via
http://kramer.ume.maine.edu/~panda/phyast/aquarius/figure2.jpg.
A plot of the red shift distribution for all rich galaxy clusters that
now have at least one measured red shift within the wedge of space
outlined by a strip of the sky containing the Aquarius supercluster.
The angular coordinate is measured from the northern end of the strip.
Triangle symbols represent clusters with magnitude of tenth brightest
galaxy less than 18.4, for which this region is 80% complete. Crosses
represent a few fainter clusters that now have measured red shifts. The
region near velocity 33,000 km/sec and an angle of 10 degrees contains
seven clusters in a clump that is about 200 times the average density
of such clusters. The region from v = 24,000 to v = 48,000 contains 23
clusters within a 200 x 150 x 1000 million light-years volume and has
an overdensity factor of about 10.
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