Press Release

Early results from the Sloan Digital Sky Survey: From under our nose to the edge of the universe

By SpaceRef Editor

June 5, 2001

Filed under Astrophysics, Sloan Digitial Sky Survey, US

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.

Quasar Record-Breakers

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.Ý”It is exciting to see this partnership work just the way we
dreamed it would.”

Asteroids

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,”Ý 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.ÝUltimately, 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,”Ý 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.

Images available:

Quasars:

* Redshift 6.2 quasar

http://www.sdss.org/news/releases/20010605.edr.img1.html

* Spectra of record-breaking quasars

http://www.sdss.org/news/releases/20010605.edr.img2.html

* Spectrum of a typical QSO

http://www.sdss.org/news/releases/20010605.edr.img3.html

* Graph of quasars discovered over time

http://www.sdss.org/news/releases/20010605.edr.img4.html

* Age of universe for the most distant quasars

http://www.sdss.org/news/releases/20010605.edr.img5.html