Princeton Weekly Bulletin January 11, 1999
Graduate student finds farthest quasars
Sloan Digital Sky Survey's new mapping technology produces unexpectedly prompt results
Just a few months after the Sloan Digital Sky Survey's new sky-mapping technology began operation, scientists discovered the two most distant quasars ever observed.
Princeton graduate student Xiaohui Fan broke the news at a December 4 collaboration meeting at the Department of Energy's Fermi National Accelerator Lab.
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Michael Strauss, assistant professor of astrophysical sciences, Fan's faculty adviser, described the discovery.
"Xiaohui and I were in the basement of Peyton Hall, operating the 3.5-meter diameter telescope at Apache Point Observatory over the internet. It was 1:30 a.m. on Thanksgiving morning, and we only had about half an hour of observing time left. Xiaohui suggested observing one of our last promising high-redshift quasar candidates before we finished up. As soon as we saw the spectrum, we knew we had a record-breaking quasar."
Redshift measures distance
Quasars are compact, luminous objects thought to be powered by supermassive black holes. Redshift is the amount by which light is shifted toward the red end of an object's spectrum by the expansion of the universe. Astronomers use redshift as a measure of the distance of celestial objects: the higher the redshift, the greater the distance and the younger the universe when the light was emitted.
The newly discovered quasars have redshifts of 5.0, 4.9 and 4.75, two surpassing the previous recordholder: a 4.89-redshift quasar discovered in 1991 by Donald Schneider, now of Pennsylvania State University; Maarten Schmidt of California Institute of Technology; and James Gunn, Princeton's Eugene Higgins Professor of Astronomy, who built both the camera and the spectrograph used by the Sky Survey. These quasars mark an epoch when the universe was less than a billion years old and a sixth of its current size.
The Sky Survey's telescope at Apache Point, N.M., revealed the three new quasars a short time after first light this past May. Members of the team were still calibrating instruments, building data archives and installing a new monitor telescope when the discovery data were taken in September.
"These results are revolutionary," said Schneider. "Looking at only one percent of the data and using prototype software and only a preliminary calibration to evaluate it, we have achieved an 70 percent success rate in identifying bright high-redshift quasars. At the current rate of discovery, by the end of the survey we should find more than 500 quasars with redshift greater than 4.75."
Asteroids to hybrid galaxies
Imaging a slender slice of sky during its trial runs, the Sky Survey's wide-field telescope electronically recorded millions of objects. The images depicted a broad range of objects -- from nearby asteroids between Mars and Jupiter to mysterious hybrid galaxies -- and provided scientists with an astonishing array of data to analyze.
Fan and Fermilab scientists Heidi Newberg and Brian Yanny analyzed the images, using software developed largely by Robert Lupton of Princeton's senior technical staff. Fan and colleagues used highly accurate measurements of the relative brightness of objects as seen through different filters to select a few dozen that might be high-redshift quasars, then they used a separate 3.5-meter telescope at Apache Point to verify that they had detected 12 new quasars at redshifts above 3.6.
Presenting the data, Fan said, "There is every reason to believe that in years to come there will be even more exciting results from this survey. We have found the most distant quasars quite easily, after people have been searching for them for decades. We can look forward to many years of good science."
100 times larger
The ultimate goal of the Sky Survey, an international collaboration of more than 100 scientists and engineers, is to map one quarter of the sky and create a systematic, three-dimensional picture of the universe 100 times larger than in previous surveys.
Participating institutions in the Sky Survey include Princeton and Johns Hopkins universities, the universities of Chicago and Washington, Fermilab, the Institute for Advanced Study, Japanese Participation Group and U.S. Naval Observatory. Apache Point Observatory is owned by the nonprofit Astrophysical Research Consortium of seven research institutions and operated by New Mexico State University. Funding has been provided by the Alfred P. Sloan Foundation, Sky Survey member institutions, National Science Foundation, National Aeronautics and Space Administration, and U.S. Department of Energy.