Dawning of Light in the Universe Discovered by Astronomers

An international team of 28 scientists from the Sloan Digital Sky Survey, including two Penn State astronomers, has found evidence that the most distant object yet detected may be one of the universe's first powerful sources of light, solving a mystery that had eluded scientists for nearly four decades—when did light first start to break through the dark clouds that filled the early universe.

The object is a quasar — a type of galaxy that produces intensely luminous radiation from the violent destruction of some of its stars by its massive central black hole. This particular quasar is one of several very distant such objects whose discovery was announced in June 2001 by Penn State Professor of Astronomy and Astrophysics Donald Schneider, who has been chair of the Sloan Digital Sky Survey quasar science group since its inception in the early 1990s. Schneider, along with Penn State postdoctoral fellow Gordon Richards, is a coauthor of the recent mystery-solving analysis of the quasars' spectra from new, high-quality observations with the Keck telescope in Hawaii. The study has been submitted for publication in the Astronomical Journal.

"The conditions of the intergalactic material near the most distant of these quasars is strikingly different from everywhere else we have looked outside the galaxies, from close to our galaxy in space and time to as far away and long ago as possible with the most powerful telescopes. A significant fraction of the material near the distant quasar appears to include some of the dark clouds from the earlier epoch in the history of the universe known as the Dark Ages," Schneider explains.

The most distant quasar yet discovered sits at the astronomical zip code known as "redshift 6.28," which is so far away that the object's light started on its journey toward Earth when the universe was only about 5 percent of its current age. To learn what space was like that long ago, the Sloan team used the quasar's powerful light much as doctors use X-rays to reveal structures inside the human body. The material in space absorbs some of the quasar's light, leaving the material's imprint on spectrum of the light received at Earth from the quasar.

Before the Sloan team's discovery, everywhere astronomers had looked at intergalactic space they could find nothing but hot plasma — electrons, protons, and atomic nuclei at very high temperatures. Yet much farther back in time — within a million years after the explosive birth of the universe known as the Big Bang about 13 billion years ago — astronomers agree that the universe already had cooled enough for the electrons and nuclei to combine to form atoms, the vast majority of which were hydrogen atoms.

The period before stars began to shine is known as the "Dark Ages" of the universe. "Light can penetrate through plasma, but not through clouds of hydrogen atoms, which are extremely effective at absorbing ultraviolet radiation," Richards says.

Shortly after the discovery of the first quasar in the early 1960s, two graduate students at Caltech, Bruce Peterson and James Gunn, who now is a professor at Princeton University and the Project Scientist for the Sloan Survey, proposed a way to examine the state of the intergalactic material in the early universe. This test, known as the "Gunn-Peterson" effect, works by looking for a distinctive pattern of radiation absorption in light from distant quasars. Astronomers were surprised by early Gunn-Peterson observations that indicated that the intergalactic medium today is primarily a hot plasma, and that this was the condition of the universe back to the time of the most distant known objects. This result implied that an enormous amount of energy had been released at some unknown point in the past. The recent Keck observations have, for the first time, revealed absorption of radiation at the exact locations in the spectrum predicted by Gunn and Peterson.

"Although we are not yet certain of the source of this energy that reheated the intergalactic medium, for the first time we know when it occurred," remarked Richards.

The team's analysis of the light from the farthest quasar reveals its region of space at redshift 6.28 contains a mixture of both the cold hydrogen atoms of the very early universe and the hot plasma that permeates our universe today. In contrast, the spectrum of the second most distant quasar, at redshift 6.0 when the universe was less than 100 million years older than the time when we view the most distant object, shows little indication of the Gunn-Peterson effect. These bookends of time provide the first evidence of the beginning of the era known as the "cosmic renaissance," when light first began burning through the dense clouds of hydrogen atoms from the earlier epoch known as the "Dark Ages," which shrouded and blocked the light from the very first stars. "To rip apart or ionize hydrogen atoms, you need very powerful ultraviolet radiation like that produced by quasars and massive stars many times larger than our Sun," Richards explains.

"This heating of the universe had to have begun even earlier than our measurements have revealed, but for the first time we now have a handle on when this occurred," Schneider says. Astronomers will need to observe quasars at even greater distances from Earth to confirm this discovery and to learn whether the dark ages ended at about the same time in all parts of the universe. "We now have detected evidence of one of the major events in the history of the universe, which is one of the most fundamental values used in models of the evolution of its large-scale structure," Schneider says. "The next goal is to find a number of these very distant objects to be certain that this single observation is not an unusual feature of this specific object but is a general property of the intergalactic medium at that time, and we expect the Sloan survey will identify a number of such quasars in the next few years."

The Sloan Digital Sky Survey, a large international effort, aims to observe 100,000 quasars, measure the distances to a million galaxies, and to produce a comprehensive digital map of the sky during the next five years. More information about the Sloan Digital Sky Survey is on the web at http://www.sdss.org

CONTACT:

Gordon Richards, 814-863-6091, gtr@astro.psu.edu

Donald Schneider, 814-863-9554, dps@astro.psu.edu

Barbara K. Kennedy, 814-863-4682 or 814-863-8453, science@psu.edu