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ASTRONOMERS
MAP OUT LARGEST STRUCTURE IN DISTANT UNIVERSE
By reading the light from the
fiery heart of unimaginably remote galaxies, astronomers have
discovered evidence for an immense concentration of galaxies
over 6.5 billion light years away in the largest known group
of quasars, possibly the largest structure anywhere in the
observable universe. The galaxies were revealed by light they
absorbed from the spectra of even more distant quasars which
are located behind the large quasar group.
Such huge structures in space
are very rare. The universe at the distance of the cluster is
seen at only about a third of its present age. If the
concentration of galaxies and quasars is caused by a larger
than usual amount of matter in the area, traditional theories
of the evolution of the universe have difficulty explaining
how gravity could pull extremely massive structures together
over such a large distance, in such a relatively short time.
Measuring the actual amount of matter connected with quasars
and galaxies at such great distances is crucial to find out
whether the theories still hold.
The massive galaxy
concentration is located just south of the heart of the
constellation Leo the Lion, in a region spanning up to two by
five degrees -- or forty times the area of the full moon. It
is contained within a large quasar group, which measures about
600 million light years across, making it the largest
structure known in the early universe. (A light year is the
distance traveled by light in a year, about six trillion
miles).
"A successful theory has
to explain the extremes. Bizarre things like this huge
supercluster present a unique opportunity to measure how well
quasars and galaxies reveal the mass in such a big region of
space, which can then be connected to predictions from
theories," said Dr. Gerard Williger of the National
Optical Astronomy Observatories, who is currently stationed at
NASA's Goddard Space Flight Center in Greenbelt, Md.
"The light we are
presently observing from this large quasar group had to cross
such a vast distance to reach us that it actually left the
group before the Earth was formed," said Williger.
"We see these galaxies as they existed billions of years
ago. The amount of matter connected with quasars and galaxies
at such distances and distant times in the past is probably
not be the same as we would measure in the local universe
today, so it's very important to find out how much mass we are
actually looking at in the supercluster. The first step is to
look for signs of extra galaxies in the area, and now we have
evidence for a surplus of galaxies."
This research will be presented
January 8 during the winter meeting of the American
Astronomical Society in San Diego, California, and is being
prepared for submission to the Astrophysical Journal.
The galaxy cluster is outlined
by an unusually large number of quasars - hence the
designation "large quasar group". Quasars are
galaxies with bright cores, probably powered by giant black
holes. Some quasars shine with the light of a trillion suns,
and astronomers use them as cosmic beacons to probe the
remotest reaches of the universe. This large quasar group
holds 18 quasars in a swath of space roughly a half billion
light years across. Normally, only about two to three quasars
would be expected in a region this size if there were no
galaxy cluster present.
In addition to the quasars,
Williger, Dr. Luis Campusano (University of Chile), Dr. Roger
Clowes and graduate student Chris Haines (University of
Central Lancashire, England) used the 4-meter (159 inch)
telescope at the National Science Foundation's Cerro Tololo
Inter-American Observatory in Chile to find a large number of
clouds of gas in the same area, which are thought to originate
in the haloes surrounding galaxies. Those galaxies themselves
are faint and have not yet been directly seen. However,
magnesium atoms in the halo gas of the galaxies absorb light
coming from quasars behind the cluster. This produces
"shadows" in front of the quasars, thus revealing
the existence of the galaxies. So far, 11 such faint absorber
galaxies have also been found in the same space as the large
group of quasars, which is nearly three times more than
expected.
The group cautioned these
results still need to pass the test of peer review, and also
be confirmed with a larger data sample, ideally toward other
large quasar groups.
The discovery of the light
absorption by the galaxies puts "flesh" on the
"skeleton" of the group outlined by the quasars. It
allows Williger, Campusano, Clowes and their collaborators to
study the relationship between galaxies and quasars in a large
group.
Quasars can form from merging
galaxies, which are common in the centers of clusters, and
also may form from the compression of gas where the edges of
clusters of galaxies come together. In one model, quasars in
the distant past should then be found both in the centers and
on the edges of galaxy clusters. At later times, as the gas in
the centers of the galaxy clusters forms into galaxies, most
of the remaining gas should be found on the outskirts of
galaxy clusters, and quasars should be found mostly on their
peripheries. Information about the galaxies in the large
quasar group will help to determine whether this view is
correct.
"At the moment, computer
simulations of the formation of large structures in the
universe are just starting to deal with sizes as large as this
large quasar group. These groups are fantastic laboratories
for the studying the formation and evolution of quasars and
galaxies," said Williger.
MEDIA
CONTACT LIST
American
Astronomical Society Meeting Press Room (7 - 11 January 2001)
619
908 5057
619 908 5040
619 908 5041
Doug Isbell/NSF NOAO PIO/National Solar Observatory
520 318 8214 (at AAS press room 7 - 11 January 2001)
Amber Jones/NSF PIO/NSF Headquarters
703 306 1070 (at AAS press room 7 - 11 January 2001)
Bill Steigerwald/NASA Technical Writer/Goddard Space Flight
Center
(301) 286 5017 (at AAS press room 7 - 11 January 2001)
Nancy Neal/NASA Origins program PIO/Goddard Space Flight Center
301 286 0039
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