Finding ancient galaxies in quasar 'shadows'

By Diana Steele
News Office

Using conventional astronomical methods, astronomers can "see" only galaxies that are relatively nearby; those at great distances are too faint to be seen by even the most powerful telescopes. Now, using an unconventional method, astrophysicist Jean Quashnock, Research Scientist in the Enrico Fermi Institute, has found a way to detect the signatures of galaxies that existed 13 billion years ago, just two billion years after the birth of the universe.

Quasars, or quasi-stellar objects, can be seen nearly as far away as the edge of the visible universe, because they are incredibly bright and point-like. By using "shadows" in the light from quasars, Quashnock is mapping the large-scale structure of the ancient universe. He presented his research on Jan. 10 at the 191st meeting of the American Astronomical Society in Washington, D.C.

"We can trace the evolution of structure in the universe over cosmic time, from its teenage years through adulthood," said Quashnock. "Using this method, we see more than 60 percent of the visible universe, albeit sparsely sampled."

The light from quasars shines through the intervening gas and dust that comprise galaxies and gas clouds amidst the voids of outer space, before it reaches telescopes here on Earth. By looking at the spectrum of light from a quasar, astronomers can "see" these intervening structures because they absorb some of the light in particular wavelengths. The spectrum shows absorption bands wherever the light has been absorbed.

By analyzing the spectrum, Quashnock has begun to map the large-scale structure in the distant -- and ancient -- universe.

Astronomers already have been able to map some of the nearby galaxies, which show clusters and superclusters of galaxies appearing on large scales in intriguing shapes some have dubbed the "Great Wall" and the "Stick Man." Now Quashnock has found that this superclustering of galaxies -- like the "Great Wall" -- already existed in the early universe, as traced by the spatial distribution of the gas clouds at cosmological distances.

Quashnock's work is beginning to fill in the gap between scientists who are looking at the birth of the universe by studying the cosmic microwave background -- which traces the structure of the universe only 300,000 years after the Big Bang -- and astronomers who are mapping more and more of the nearby universe. At Chicago, astrophysicists Stephan Meyer, John Carlstrom and Mark Dragovan are working on experiments in detecting the microwave background, and Donald York, Richard Kron and others are involved with the Sloan Digital Sky Survey, which, when it gets underway in the next two years, will be the most comprehensive map ever made of the nearby universe.

Quashnock is analyzing the spectra of 500 quasars in a catalogue compiled from the literature by York, the Horace B. Horton Distinguished Service Professor in Astronomy & Astrophysics, and his colleagues. The analysis was performed using statistical techniques developed by Michael Stein, Professor in Statistics.

Quashnock's research is funded by NASA and the National Science Foundation.