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Tuesday, 9 January, 2001, 10:35 GMT
Huge galaxy grouping detected
Supercluster Nasa
Thousands of galaxies in the supercluster stand behind red and yellow-coloured foreground stars
Astronomers have identified a supercluster of galaxies that could be the biggest structure anywhere in the observable Universe.

The grouping, which is in the constellation of Leo, is thought to be about 500 million light-years across and about 6.5 billion light-years from Earth. Because in astronomical terms distance equals time, scientists are seeing the supercluster when the Universe was perhaps one-third the age it is now.

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

Dr Gerard Williger
The concentration of galaxies was revealed by the light it absorbed from even more distant quasars located behind the supercluster.

Quasars, "active" galaxies that have very bright cores, probably powered by giant matter-sucking black holes, have become useful tools for probing deep space. They help scientists to map the spatial distribution of galaxies, a task that is vital in developing coherent theories to explain why the matter in the Universe is spread out in the way it is.

The supercluster itself also contains an unusually large number of quasars of its own. It has 18 in a swath of space astronomers would normally expect to find only about two to three if there was no galaxy cluster present.

Surplus galaxies

The international team of astronomers that made the latest discovery said the supercluster posed some questions for current cosmological theories.

Supercluster Nasa
Quasars are used to probe the intervening space to Earth
If the concentration of galaxies was caused by a larger than usual amount of matter in the area, traditional theories of the evolution of the Universe would have difficulty explaining how gravity could pull extremely massive structures together over such a large distance, in such a relatively short time, they said.

"A successful theory has to explain the extremes," said Dr Gerard Williger, from Nasa's Goddard Space Flight Center and the National Optical Astronomy Observatories in Arizona, US.

"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. 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 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."

Supercluster Nasa
The grouping is in the constellation of Leo
The research was presented on Monday to a meeting of the American Astronomical Society in San Diego, California, and is being prepared for submission to the Astrophysical Journal.

Working with Luis Campusano of the University of Chile and Roger Clowes and Chris Haines of the University of Central Lancashire in the UK, Williger stressed that the research needed to be confirmed with more data.

Quasar facts
Enormously bright objects at the edge of the Universe
Appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar)
Some quasars shine with the light of a trillion suns
A type of active galaxy powered by supermassive black hole
Quasars can show up intervening galaxies
Used to probe the early Universe
"We haven't found anything bigger in the literature," he said. "And nobody has brought anything bigger to our attention, but I don't want to go out on a limb and say this is absolutely the biggest because then that surely means somebody is going to come up and say, "but wait ...'"

Scientists mapping the Universe see galaxies as part of groups, clusters and superclusters. These structures may be part of filaments, sheets and walls that enclose huge voids.

Our own galaxy, the Milky Way, is a tiny feature in a relatively small supercluster of galaxies about 50 million light-years in extent. A light-year is 9.5 trillion km (6 trillion miles), the distance light travels in a year.

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