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08/06/2001 07:00:00

Astronomers detect evidence of time when universe emerged from "Dark Ages"

Astronomers at the California Institute of Technology announced today the discovery of the long-sought "Cosmic Renaissance," the epoch when young galaxies and quasars in the early universe first broke out of the "Dark Ages" that followed the Big Bang.

"It is very exciting," said Caltech astronomy professor S. George Djorgovski, who led the team that made the discovery. "This was one of the key stages in the history of the universe."

According to a generally accepted picture of modern cosmology, the universe started with the Big Bang some 14 billion years ago, and was quickly filled with glowing plasma composed mainly of hydrogen and helium.

As the universe expanded and cooled over the next 300,000 years, the atomic nuclei and electrons combined to make atoms of neutral gas. The glow of this "recombination era" is now observed as the cosmic microwave background radiation, whose studies have led to the recent pathbreaking insights into the geometrical nature of the universe.

The universe then entered the Dark Ages, which lasted about half a billion years, until they were ended by the formation of the first galaxies and quasars. The light from these new objects turned the opaque gas filling the universe into a transparent state again, by splitting the atoms of hydrogen into free electrons and protons. This Cosmic Renaissance is also referred to by cosmologists as the "reionization era," and it signals the birth of the first galaxies in the early universe.

"It is as if the universe was filled by a dark, opaque fog up to that time," explains Sandra Castro, a postdoctoral scholar at Caltech and a member of the team. "Then the fires—the first galaxies—lit up and burned through the fog. They made both the light and the clarity."

The researchers saw the tell-tale signature of the cosmic reionization in the spectra of a very distant quasar, SDSS 1044-0125, discovered last year by the Sloan Digital Sky Survey (SDSS). Quasars are very luminous objects in the distant universe, believed to be powered by massive black holes.

The spectra of the quasar were obtained at the W. M. Keck Observatory's Keck II 10-meter telescope atop Mauna Kea, Hawaii. The spectra show extended dark regions, caused by opaque gas along the line of sight between Earth and the quasar. This effect was predicted in 1965 by James Gunn and Bruce Peterson, both then at Caltech. Gunn, now at Princeton University, is the leader of the Sloan Digital Sky Survey; Peterson is now at Mt. Stromlo and Siding Spring observatories, in Australia.

The process of converting the dark, opaque universe into a transparent, lit-up universe was not instantaneous: it may have lasted tens or even hundreds of millions of years, as the first bright galaxies and quasars were gradually appearing on the scene, the spheres of their illumination growing until they overlapped completely.

"Our data show the trailing end of the reionization era," says Daniel Stern, a staff scientist at the Jet Propulsion Laboratory and a member of the team. "There were opaque regions in the universe back then, interspersed with bubbles of light and transparent gas."

"This is exactly what modern theoretical models predict," Stern added. "But the very start of this process seems to be just outside the range of our data."

Indeed, the Sloan Digital Sky Survey team has recently discovered a couple of even more distant quasars, and has reported in the news media that they, too, see the signature of the reionization era in the spectra obtained at the Keck telescope.

"It is a wonderful confirmation of our result," says Djorgovski. "The SDSS deserves much credit for finding these quasars, which can now be used as probes of the distant universe—and for their independent discovery of the reionization era."

"It is a great example of a synergy of large digital sky surveys, which can discover interesting targets, and their follow-up studies with large telescopes such as the Keck," adds Ashish Mahabal, a postdoctoral scholar at Caltech and a member of the team. "This is the new way of doing observational astronomy: the quasars were found by SDSS, but the discovery of the reionization era was done with the Keck."

The Caltech team's results have been submitted for publication in the Astrophysical Journal Letters, and will appear this Tuesday on the public electronic archive, http://xxx.lanl.gov/list/astro-ph/new.

The W. M. Keck Observatory is a joint venture of Caltech, the University of California, and NASA, and is made possible by a generous gift from the W. M. Keck Foundation.

Written by Robert Tindol