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GIANT
GALAXY STRING DEFIES MODELS OF HOW UNIVERSE EVOLVED
Wide-field
telescope observations of the remote and therefore early Universe,
looking back to a time when it was a fifth of its present
age (redshift = 2.38), have revealed an enormous string of
galaxies about 300 million light-years long. This new structure
defies current models of how the Universe evolved, which can't
explain how a string this big could have formed so early.
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Image
1
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The
string is comparable in size to the "Great Wall"
of galaxies found in the nearby Universe by Dr. John Huchra
and Dr. Margaret Geller in 1989. This is the first time astronomers
have been able to map an area in the early Universe big enough
to reveal such a galaxy structure.
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Image
2
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The
string was discovered by Dr. Povilas Palunas (University of
Texas, in Austin, Texas), Dr. Paul Francis (Australian National
University, Canberra, Australia), Dr. Harry Teplitz (California
Institute of Technology in Pasadena), Dr. Gerard Williger
(Johns Hopkins University, Baltimore, Md.), and Dr. Bruce
E. Woodgate (NASA Goddard Space Flight Center, Greenbelt,
Md.). The initial observations were made with the 4-m (159-inch)
Blanco Telescope at the National Science Foundation's Cerro
Tololo Inter-American Observatory in Chile, and confirmed
with the 3.9-m (154-inch) Anglo-Australian Telescope at Siding
Spring Observatory in eastern Australia. The team presents
its finding today at the American Astronomical Society meeting
in Atlanta, Georgia, and a paper describing this work will
appear in the Astrophysical Journal in February.
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Image
3
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The
string lies 10,800 million light-years away in the direction
of the southern constellation Grus (the Crane). The distance
light travels in a year, almost six trillion miles or 9.5
trillion km., is one light-year, so we see the string as it
appeared 10.8 billion years ago. It is at least 300 million
light-years long and about 50 million light-years wide. (Refer
to Movie 1 and Images 3 and 4 for an artist's concept of the
string.) The astronomers have detected 37 galaxies and one
quasar in the string, but "there are almost certainly
far more than this," said Palunas. "The string probably
contains many thousands of galaxies." (Refer to Image
1 for an artist's concept of these galaxies, and to Image
5 for a plot of their locations on the sky.)
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Image
4
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"We
are seeing this string as it was when the Universe was only
a fifth of its present age," said Woodgate. "That
is, we are looking back four-fifths of the way to the beginning
of the Universe as a result of the Big Bang."
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Image
5
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The
team compared their observations to supercomputer simulations
of the early Universe, which could not reproduce strings this
large. "The simulations tell us that you cannot take
the matter in the early Universe and line it up in strings
this large," said Francis. "There simply hasn't
been enough time since the Big Bang for it to form structures
this colossal".
"Our
best guess right now is that it's a tip-of-the-iceberg effect,"
he said. "All we are seeing is the brightest few galaxies.
That's probably far less than 1% of what's really out there,
most of which is the mysterious invisible dark matter. It
could be that the dark matter is not arranged in the same
way as the galaxies we are seeing." Recently, evidence
has accumulated for the presence of dark matter in the Universe,
an invisible form of matter only detectable by the gravitational
pull it exerts on ordinary matter (and light). There are many
possibilities for what dark matter might be, but its true
nature is currently unknown.
In
recent years, Francis explained, it had been found that in
the local Universe, dark matter is distributed on large scales
in very much the same way the galaxies are, rather than being
more clumpy, or less. But go back 10 billion years and it
could be a very different story. Galaxies probably form in
the center of dark matter clouds. But in the early Universe,
most galaxies had not yet formed, and most dark matter clouds
will not yet contain a galaxy.
"To
explain our results," said Francis, "the dark matter
clouds that lie in strings must have formed galaxies, while
the dark matter clouds elsewhere have not done so. We've no
idea why this happened - it's not what the models predict."
To
follow up this research, the astronomers say, the next step
is to map an area of sky ten times larger, to get a better
idea of the large-scale structure. Several such surveys are
currently under way. The research was funded by NASA and the
Australian National University.
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