The most distant object ever found by probing the universe with x-rays has been discovered by an international team of astronomers using the x-ray satellite, ROSAT. The object, a quasar whose 12-billion-year-old radiation has been speeding toward Earth since the universe was in its infancy, was detected with the deepest x-ray exposure ever made, according to a paper to be published in the April issue of the Astronomical Journal.
"This quasar is one of the faintest x-ray sources ever detected," says Donald Schneider, associate professor of astronomy and astrophysics at Penn State and an author of the paper describing the discovery.
Quasars, which are the most luminous objects in the universe and are thought to contain the black-hole seeds from which all galaxies formed, are among the most distant objects known. "A quasar produces about 100 times as much energy as our entire galaxy but its volume typically is less than the size of our solar system," explains Schneider. Because radiation from quasars takes billions of years to reach the Earth, scientists see distant quasars as they were billions of years ago and use them as probes to study the early history of the universe.
The researchers discovered the distant quasar, christened "RX J105225.9+571905," by pointing ROSAT's High-Resolution Imager (HRI) x-ray camera at a patch of sky for about a million seconds--a very long time by astronomers' standards--in a study known as the ROSAT Deep Survey. The group, including astronomers from the United States, Germany, and Italy, obtained enough time on the telescope to look deeper into space in x-rays than anyone ever had done before. "The purpose of this x-ray survey was to determine the nature of faint x-ray sources, says Guenther Hasinger, director of the Astrophysical Institute in Potsdam, Germany. "It surprised us by revealing one of the most distant objects known."
In addition to Schneider and Hasinger, other astronomers involved with this work are Maarten Schmidt at Caltech, Ingo Lehmann at Potsdam, James Gunn at Princeton University, Riccardo Giacconi at the European Southern Observatory, J. Trümper at the Max Plank Institute, and Gianni Zamorani in Bologna.
The enormous energies released by a quasar result from matter tumultuously tumbling into its central black hole during the initial formation of a galaxy, many astronomers believe. Quasars are observed to be plentiful early in the history of the universe but to be quite rare today. The black holes still exist, but have had time by now to devour all the matter within their reach. "There is good evidence that a black hole resides at the center of our Milky Way galaxy, but we do not see a quasar because there is no material currently falling into the black hole," Schneider says.
In order to gauge the distance from Earth to the objects revealed by ROSAT's x-ray observatory, the scientists had to study them with one of the world's largest optical telescopes, the Keck telescope in Hawaii. "The X-ray satellite reveals that there is an X-ray source in this part of the sky, but it doesn't tell us what it is and it doesn't tell us how far away it is — we determine that from its visible light," said Hasinger. The most distant objects are speeding away the fastest, so their visible light appears to be more "redshifted," or skewed toward the red end of the spectrum.
"This quasar is so faint in visible light, it is near the limit of what the giant Keck telescope can measure and — because there are large numbers of optical objects at these brightnesses — our x-ray resolution had to be very accurate in order to determine which object our x-ray source matches in the Keck optical image," Hasinger explains. The German team, led by Hasinger, developed the techniques that made it possible for the astronomers to assign very accurate celestial positions to x-ray sources in the ROSAT survey. "The HRI now yields positions that are accurate to about 2 arc seconds — less than one-one-thousandth of a degree — whereas before we were getting positions accurate only within 5 to 10 arc-seconds."
The researchers discovered that the new quasar is located at redshift 4.45, which is so far away that we see it as it appeared when the universe was only about nine percent of its current age. "Most of the other objects in the survey turned out to be galaxies or quasars much closer to Earth, with redshifts less than two," Schneider says.
While a few objects more distant than the new quasar have been discovered, this is the most distant object ever discovered in an x-ray survey, according to the astronomers. Neil Brandt, assistant professor of astronomy and astrophysics at Penn State, has been investigating the x-ray properties of distant quasars. "Altogether, there are about 100 high-redshift quasars now known and x-rays have been detected coming from only about 9 of them," Brandt says. "We suspect the others are producing x-rays that are just too weak to be detected with our current instruments and typical exposure times."
Astronomers may have to wait only until the end of this year to get a next-generation x-ray camera that will produce much sharper images than are possible now. The "AXAF Charge-coupled device Imaging Spectrometer" (ACIS), which is scheduled for a Space Shuttle launch this fall, is one of the instruments on the world's most powerful X-ray-astronomy observatory, NASA's Advanced X-ray Astrophysics Facility (AXAF). AXAF will be the third of NASA's "Great Observatories" to be launched, following the Hubble Space Telescope, which detects ultraviolet, visible, and infrared rays, and the Compton Gamma-Ray Observatory, which detects gamma rays. "Among the wonders the ACIS camera is designed to see is the early growth of the seeds of quasars in the infant universe," says Gordon Garmire, the Evan Pugh Professor of Astronomy and Astrophysics at Penn State and the principal investigator who conceived and designed the camera.
"This research holds great promise for our future work at Penn State," Schneider comments. "With Dr. Garmire's ASIS camera and the next generation of large optical telescopes including the new Hobby-Eberly Telescope, in which Penn State is a major partner, we expect to be able to discover, identify, and locate many more very faint x-ray objects," he says. "We will be quite surprised if a number of them are not much closer to the beginning of time than the quasar found in our current survey."
This research was supported by the National Science Foundation, the National Aeronautics and Space Administration (NASA), the German Center for Space Research, and the Italian Space Agency.