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Sky Survey Racks Up Record-Setting Quasars

CHICAGO--The Sloan Digital Sky Survey, not even yet in full operation, has already bagged three of the four most distant quasars ever seen, including a new record-holder. It has also found another nine of these distant beacons, thought to be the cores of young galaxies set ablaze by mysterious central engines, team members announced on 4 December near here, at a collaboration meeting at Fermi National Accelerator Laboratory.

The Sloan survey aims to census about one-quarter of the entire northern sky and selected slices in the south, using an automated telescope at Apache Point Observatory in New Mexico (Science, 29 May, p. 1337). From hundreds of millions of celestial objects, special software will cull particularly interesting ones for a follow-up look with the same telescope, which will break their light into spectra, rich in information about the objects' nature and distance. For example, spectra of the million brightest galaxies in the census will determine their "redshift"--a measure of distance. That information will go toward creating a giant three-dimensional map of the sky. Researchers hope to use the thousands of quasars expected from the 5-year Sloan survey as markers of cosmic structure in the early universe and probes of the gases wafting through space over billions of light-years.

The telescope's first sweep of the sky, mostly in September, covered a narrow strip of sky along the celestial equator--just 1% of the area of the final survey. But Sloan collaborators, including Michael Strauss and Xiaohui Fan of Princeton University, have picked out 19 quasar candidates so far by analyzing the five-color images, and follow-up spectra confirmed 12 of them as actual quasars--a 70% success rate. That far exceeds the 10% success rate typical of quasar hunts, probably because the Sloan's images have more colors than most surveys produce. The farthest of the quasars, at a redshift of 5.00--corresponding roughly to 13 billion light-years away--just edged out the redshift 4.897 quasar reported in 1991.

These quasar finds--the Sloan's first harvest for science--has other astronomers giving it the thumbs-up. "This is one of things they wanted to do better than anyone else," says Charles Steidel of the California Institute of Technology. "It looks very promising."