Survey Probes Cosmos from Near to Far.

When astronomers complete the Sloan Digital Sky Survey in 2004, they will have mapped the position and brightness of more than 100 million celestial objects over one-quarter of the sky. They will also have measured the distance from Earth to more than 1 million galaxies and quasars.

Early findings from this most mammoth of sky surveys, based at Apache Point Observatory in New Mexico, are already yielding a trove of findings. Among the gems are the two most distant quasars known and new findings about the large-scale clumping of galaxies. In our own solar system, the survey has put the spotlight on asteroids.

"A first glance at the data shows that the survey is everything we hoped for and more," says Sloan researcher Alex Szalay of Johns Hopkins University in Baltimore. He and other astronomers presented their findings June 5 in Pasadena, Calif., at a meeting of the American Astronomical Society.

The team announced the discovery of three distant quasars, including two that break the previous distance record. Xiaohui Fan of the Institute of Advanced Study in Princeton, N.J., and his colleagues identified one quasar residing at 11.95 billion light-years from Earth and another, the new champ, at 12.04 billion light-years. The light now being detected left the quasars when the cosmos was only about 800 million years old and one-seventh its current size.

Despite the extraordinary distance of these quasars, a galaxy reported last year still qualifies as the most remote object known (SN: 5/27/00, p. 340).

Detecting a distant quasar has one advantage over finding a remote but ordinary galaxy. Because a quasar is typically 100 times brighter than a galaxy, it acts as a cosmic flashlight, illuminating the intergalactic space between it and Earth. By studying the absorption of light from the record-breaking quasars when it journeyed through gas clouds and galaxies, astronomers can discern the composition of the cosmos at times earlier than ever before, notes Sloan collaborator Donald P. Schneider of Pennsylvania State University in University Park.

Moreover, says Richard G. McMahon of the University of Cambridge in England, astronomers hold that quasars reside at the core of massive galaxies and are powered by black holes more than 100 million times as massive as the sun. The newly found quasars, therefore, "push back the time when the first [massive] galaxies formed," he notes.

After analyzing about 150,000 galaxies whose distances were measured at the Apache Point Observatory, the Sloan astronomers say they are beginning to learn how different types of galaxies--those with different shapes, colors, or brightness--arrange themselves. The survey has already confirmed, for example, that massive galaxies cluster more strongly than lightweight ones do.

"We're seeing how the environment plays a role in how galaxies form," says Sloan researcher Joshua Frieman of the Fermi National Accelerator Laboratory in Batavia, Ill. For example, galaxies born in clusters are more likely to collide and may therefore evolve differently from galaxies born in isolation.

In another part of the Sloan study, astronomers have examined the arrangement of about 1.5 million of the 8 million galaxies that the survey has now imaged, the largest galaxy map ever compiled. For the first time, astronomers can determine the extent to which galaxies congregate over distances as large as 300 million light-years, says Frieman.

The large-scale clustering pattern seen by Sloan matches that predicted by the leading theory for the evolution of the universe, Szalay reports. Known as inflation, this theory posits that when the universe was just a tiny fraction of a second old, it underwent an enormous growth spurt. Recent findings from other galaxy surveys also support the theory, Szalay notes.

Many astronomers consider the asteroids detected in the Sloan images to be a nuisance, requiring observers to more carefully identify galaxies. But "one man's contaminant is another man's data," notes Thomas Quinn of the University of Washington in Seattle.

Quinn says the Sloan images, which can detect asteroids as small as 300 meters across, indicate that there are fewer members of the asteroid belt smaller than 4 kilometers in diameter than had been previously calculated. Alan W. Harris of NASA's Jet Propulsion Laboratory in Pasadena says that Quinn's count fits well with current estimates "and must be regarded as a confirmation, not a surprise."

Quinn suggests that if there are fewer small asteroids near Earth than expected, they would pose less of a threat of collision with our planet. Harris notes that it would be more accurate to examine the near-Earth population directly.

After studying the colors of several thousand asteroids, Quinn and his colleagues have reported that the asteroid belt can be clearly divided in two: an outer belt of carbon-rich asteroids centered at 3.2 astronomical units (AU) from the sun and an inner belt of silicate-rich asteroids centered at 2.8 AU. One AU is the distance between Earth and the sun.

Planetary scientists say that this general division, which has been recognized for 2 decades, reflects the pattern established during the formation of the solar system 4.5 billion years ago. Because carbon evaporates easily, carbon-rich asteroids would more likely have coalesced in the cooler, more remote regions of the disk of dust and gas that swaddled the young sun. Silicate-rich asteroids would have formed closer in, at higher temperatures.

Quinn's team has "persuasive evidence that the asteroid belt is really divided into separate belts," says Scott D. Tremaine of Princeton University. The Sloan researchers have gleaned global properties of asteroids by briefly imaging a large group of objects, he adds. "It's a new mindset," he says.