Diagram represents how galaxies that are part of the large quasar group reveal themselves by blocking some of the light from quasars behind them. The purple dot represents a quasar, a galaxy with an exceptionally bright core, possibly powered by a giant black hole, that shines with the light of a trillion Suns. The red, green, and blue lines represent light of different colors emitted by the quasar. An intervening galaxy (white swirl in middle image) absorbs light of a certain color, represented by the green line, while letting the rest of the light from the quasar pass through. Photo: NASA, Gerard Williger

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 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.

This is a composite image showing the location of the largest known quasar group and associated galaxies. The massive galaxy concentration is located just south of the heart of the constellation Leo the Lion, represented here by a line connecting the stars that comprise the constellation. The cluster spans a region up to two by five degrees -- or forty times the area of the full moon. Its location and approximate apparent size in the sky is represented by the square on the bottom left side of the image, beneath the constellation Leo. The galaxy cluster 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. Photo: National Science Foundation National Optical Astronomy Observatory, Chris Haines, Gerard Williger

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.