Astronomers map out largest structure in distant universe
NASA NEWS RELEASE
Posted: January 9, 2001
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 was 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.
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.