Early results from the Sloan Digital Sky Survey: From under our nose to the edge of the universe
Pasadena, Calif. -- Scientists of the Sloan Digital Sky Survey today (June 5) presented results based on early data from the project. The first glimpses of what will ultimately be the most comprehensive and fully digital map of the sky included identification of the two most distant objects ever observed; new light on asteroids; and the first SDSS results on the large-scale distribution of galaxies.
Sky Survey collaborators also announced the release to the worldwide astronomy community of the first large piece of the digital sky.
Dr. Donald Schneider of the Pennsylvania State University described the observation of two quasars with respective redshifts of 6.0 and 6.2, the most distant objects ever observed in the universe. The two new quasars break the previous distance record established by last year's SDSS discovery of a quasar at redshift 5.8. The data presented today also reveal still another quasar of redshift 5.8. The redshift of an object directly indicates the relative size of the universe when the light was emitted: light from a redshift 6.2 quasar left that object when the universe was 7.2 times smaller than it is today.
"Quasars are precocious galaxies whose massive black holes began accreting matter, lighting them up when the universe was less than 800 million years old," said Dr. Xiaohui Fan of the Institute for Advanced Study in Princeton, New Jersey, who led the team that identified the quasars. "They allow us to study the birth of galaxies, and the first supermassive black holes."
The SDSS's goal is the observation of 100,000 quasars. To date, collaborators have discovered over 13,000, including the four most distant objects yet seen, and 26 of the 30 most distant quasars. The intervening material between us and a distant quasar absorbs some of the quasar light and leaves its imprint in the quasar's spectrum that is observed by the SDSS, explained Dr. Donald York of the University of Chicago, the Sky Survey's founding director.
"In much the same way an x-ray reveals the inside of the human body," York said, "this spectrum reveals a line of sight across the universe, sometimes passing through hundreds of galaxies. With the SDSS spectrum of a single quasar we can study hundreds of distant galaxies, some of which are just forming."
The key to SDSS's success in quasar spotting is precision digital data in five colors, allowing astronomers to distinguish quasars from more common faint stars, explained Dr. Wei Zheng of the Johns Hopkins University, who helped develop the color-tagging technique.
SDSS scientists follow up identified high-redshift quasar candidates on larger telescopes to confirm their spectra. Astronomers took the spectra of the two record-breakers with the Astrophysical Research Consortium's 3.5 m telescope, located at the Apache Point Observatory in New Mexico, also the site of the SDSS telescope.
"The discovery demonstrates the power of the 2.5 m special-purpose SDSS telescope working in tandem with the larger 3.5 m general-purpose telescope," York said. "It is exciting to see this partnership work just the way we dreamed it would."
The SDSS collaborators presented the first clear evidence for chemical segregation in the belt of asteroids between Mars and Jupiter.
By viewing the asteroids in five color bands, explained Dr. Tom Quinn of the University of Washington, the SDSS survey can reliably separate individual asteroids into two main classes, rocky silicate asteroids and more primitive carbonaceous asteroids.
"The SDSS observations also showed that the two types of asteroids are spatially separated," Quinn said, "with the inner belt of rocky asteroids centered at about 2.8 AU from the sun and the outer belt of carbonaceous asteroids centered at about 3.2 AU. This distribution has important implications for unraveling how the solar system formed and suggests that planet migrations that seem to be common in other planetary systems have not occurred in ours."
One astronomical unit, or AU, corresponds to the distance from the sun to the earth.
The SDSS asteroid sample also turned up another surprise.
"There appear to be fewer asteroids smaller than about 4 km in diameter in the asteroid belt than we previously thought," said Dr. Zeljko Ivezic of Princeton University, the leader of the SDSS asteroid team. "Since the asteroid belt is believed to be the reservoir for Earth-crossing asteroids, the new SDSS observations suggest that future asteroid collisions with Earth may be less likely than previously believed."
The SDSS scanning technique allows only five minutes to follow celestial objects as they move across the field of view, said Dr. Robert Lupton of Princeton University. During this time asteroids appear to move a distance just 1/1000th the size of the moon, relative to the distant stars.
"Nevertheless," Lupton said, "we can reliably detect their tiny motions and even determine their orbital motions around the sun."
The asteroid observations rely on precision position-finding software developed by SDSS collaborators at the US Naval Observatory.
Large-Scale Galaxy Distribution
The early SDSS results for distribution of matter in the universe reinforce the current model of a low-density universe whose expansion is accelerating, announced Dr. Alex Szalay of the Johns Hopkins University.
"Our results fit with the distribution of matter inferred from measurements of the cosmic microwave background radiation," Szalay said.
Dr. Josh Frieman of the University of Chicago and Fermi National Accelerator Laboratory said that SDSS observations together with recent measurements of the cosmic microwave background radiation have now pinned down the distribution of matter from the relatively small scale of galaxies all the way to the limits of the observable universe.
"Our observations are consistent with the idea that our universe underwent a burst of enormous expansion when it was a fraction of a second old," Frieman said.
The SDSS sample has also begun to answer longstanding questions about the origins of galaxy types, the so-called "nature vs. nurture" debate.
"Our data show that different kinds of galaxies cluster differently, indicating that galaxies are influenced by their environment," said Professor David Weinberg of the Ohio State University.
The quality of the SDSS digital data supersedes that of previous surveys, which enables collaborators to make precise measurements of large-scale structure even though they are using only a few percent of the ultimate Survey data, explained Ryan Scranton, a graduate student at the University of Chicago who played a key role in the analysis.
Large-Scale Data Distribution
In a major milestone for astronomy, SDSS collaborators announced the release to the worldwide astronomy community of the Survey's early data. The release includes 500 square degrees of the sky, 500 Gigabytes of images, precision measurements of some 14 million objects, and spectra of 50,000 galaxies and 5,000 quasars. Ultimately, the SDSS plans to release to astronomers and the public all the data from the five-year Survey. This enormous data release--the biggest ever in astronomy--will reside in archives at both Fermilab and the Space Telescope Science Institute in Baltimore, MD.
"I expect that the discoveries by astronomers outside the SDSS collaboration, using the data archive, will ultimately dwarf those produced by the collaboration itself," said Dr. Marc Postman, of the Space Telescope Science Institute, who helped develop the archive interface.
Dr. Chris Stoughton of Fermilab explained that the software that produced the data involved more than a million lines of code written by the SDSS scientists.
"This early data release represents the work of dozens of astronomers, computer scientists and programmers working over the last decade," Stoughton said.
The key to accessing the data is the SkyServer, an interactive interface developed with the help of Dr. Jim Gray of Microsoft's Bay Area Research Center, said Hopkins' Szalay.
"With the SkyServer and the early Sloan data, any astronomer can navigate the universe and access data for over 10 million objects," Szalay said.
A version of the SkyServer designed for students and the general public is currently in development.
While the main science goal of the survey is a three-dimensional map of the universe constructed from the positions of a million galaxies, the SDSS will be the field guide to the heavens for decades, according to Dr. John Peoples Jr., the project's director.
"When it is complete, the sky will be open to professional astronomers, schoolchildren and the general public alike, day or night, rain or shine," Peoples said.
Funding for the Sloan Digital Sky Survey (SDSS) has been provided by the Alfred E. Sloan Foundation, the participating institutions, the National Aeronautics and Space Administration, The National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS is a joint project of The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Princeton University, the United States Naval Observatory and the University of Washington.