Far Out!

lights against black

Donald Schneider and his colleagues weren't expecting to spot one of the most distant objects yet discovered in the universe. They were just conducting a survey.

"We were trying to learn more about the properties of x-ray sources," says Schneider, an associate professor of astronomy and astrophysics at Penn State. X-ray sources are objects that radiate energy at x-ray wavelengths, much shorter than the wavelengths of visible light. Because the short waves are far more energetic than their visible counterparts, their sources, Schneider says, "tend to be very interesting objects."

To gather their data, Schneider and the other members of an international team of astronomers used the orbiting x-ray telescope ROSAT. Because x-rays are absorbed by cosmic gas and dust, they trained ROSAT's instruments at a clear patch of sky called the Lockman hole, one of the most transparent lines of sight in the Milky Way.

"We were allocated a total of a million seconds of viewing time, about 270 hours, split up over several years," says Schneider. With such a long exposure, they were able to peer deeper into space than anyone had ever done in the x-ray wavelength. One of the very faint objects they noticed out near the limits of the telescope's reach, it turns out, is a quasar that may be 10 billion light-years away.

Not that the team could tell this from the start. True, quasars are by far the most luminous objects in the universe. "A quasar produces about 100 times as much energy as our entire galaxy," Schneider says, "but its volume typically is less than that of our solar system." Because of its scarcely fathomable distance, however, this particular quasar could easily hide itself among much closer, much dimmer objects. "Many objects that appeared not much brighter than this one were actually stars in our own galaxy, 100 light years away instead of 10 billion," Schneider says.

To determine such an object's distance, he adds, requires an optical telescope, one built to detect visible light. "What you have to do is take a picture in the optical range," Schneider says, "and then do an overlay with your x-ray data," matching locations to make sure you have the same object. Then, you go back to the optical telescope and use spectroscopy—splitting the light into its component colors—in order to determine the "redshift." Light from an object traveling away from Earth appears more red than it would if the object were standing still, Schneider explains. The farther the object, the redder it appears.

The reason you don't just use an optical telescope in the first place, Schneider says, is that there are so many faint objects within the visible range that it would be impossible to check them all. Since there are many fewer objects that emit x-rays, scouting that range first is a way of narrowing your choices. Until now, however, characterizing x-ray sources once they're spotted has been a problem. "Positional error with x-rays is much larger than that with optical light," Schneider says. So the "error box" surrounding an x-ray object, when you do your overlay, might be large enough to hold several optical objects, preventing a one-to-one match.

In this survey, Schneider says, "Because of the high quality of the data, and painstaking care with the analysis," by German astronomer Guenther Hasinger and other European team members, "we were able to establish positions as accurately as has ever been done with x-ray data." That made Schneider's part of the job, the determination of redshift, much easier.

The newly discovered quasar is the most distant object ever found in an x-ray survey. It's impossible to state its distance in absolute terms, because of current uncertainty about the age (and so the size) of the universe, Schneider says. But its redshift of 4.45 enables the calculation that its light has been traveling toward Earth for more than 90 percent of that age. What he and his colleagues have observed, in other words, is an object that has been around since the universe itself was a toddler. "It's not that this is such a startling discovery in itself," he suggests, "but it shows that these high-redshift x-ray objects are out there. In the next five years, with improved technology, we should be able to see many more."

NASA expects to launch its Advanced X-ray Astrophysics Facility (AXAF) later this year, he notes, and this new satellite will be outfitted with a high-resolution camera designed by Penn State colleague Gordon Garmire, "that can see things ten times fainter than we've seen before. With AXAF, and with new optical telescopes like the Hobby-Eberly, in which Penn State is a major partner, we're going to be getting a much clearer picture of the very early universe."

Donald P. Schneider, Ph.D., is associate professor of astronomy and astrophysics in the Eberly College of Science, 504 Davey Laboratory, University Park, PA 16802; 814-863-9554; dps@astro.psu.edu. The ROSAT Deep Survey was supported by the National Science Foundation, NASA, the German Center for Space Research, and the Italian Space Agency.

Last Updated September 01, 1998