The Sloan Digital Sky Survey is looking deeper into space than ever before. What astronomers are finding is literally rewriting our history of the universe.
by Ellen Davis
In 1998, data collected by a telescope near Cloudcroft, New Mexico, enabled a young astronomy graduate student to find what was then the most distant object ever observed in the universe – a quasar, or black hole, that may help researchers understand how the universe was formed.
This discovery was the first of many that have come out of the Sloan Digital Sky Survey (SDSS), a seven-year effort that is looking deeper into space than ever before. The project is based at the Apache Point Observatory near Cloudcroft and involves a consortium of universities in the United States, Japan and Germany, including NMSU.
“The results of the project have so far exceeded our wildest expectations,” says Kurt Anderson, professor of astronomy at NMSU and site director of Apache Point Observatory.
Apache Point Observatory was selected as the home base for the Sloan Digital Sky Survey because of the clear dark skies and clean dry air found at its mountaintop site 9,200 feet above sea level in the Sacramento Mountains. Most members of the SDSS consortium also are members of the Astrophysical Research Consortium, which built Apache Point in the mid-1980s. This year marks the 10th anniversary of the first large instrument installed at the observatory, a 3.5-meter telescope.
The SDSS is the first major multicolor survey of the sky attempted since the 1950s, when the Palomar Sky Survey photographed the sky visible from the Palomar Observatory north of San Diego. Photos taken from that survey are still used by astronomers.
Unlike earlier surveys – which generated two-dimensional images – the Sloan Digital Sky Survey generates a three-dimensional map of the universe. The survey has two components – an imaging component that provides directions for celestial objects and a spectroscopic part that provides the redshifts for galaxies and quasars. Redshifts are light measurements that enable researchers to determine how far away objects are.
The project hopes to map, in depth and in detail, one-quarter of the entire sky, determining the positions and absolute brightnesses of more than 100 million celestial objects and measuring the distances to more than a million galaxies and 100,000 quasars. The completed survey will give astronomers a much better picture of the large-scale structure of the universe and how galaxies are distributed, in both space and time, within the universe.
The $80 million project is funded by the Alfred P. Sloan Foundation, as well as NASA, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, the Max Planck Society and participating institutions.
Nearly 20 NMSU staff members are working at Apache Point on the Sloan Digital Sky Survey. Their job involves maintaining and operating a 2.5-meter telescope that is the most complex imaging instrument ever developed for astronomy. The telescope includes the world’s largest format imaging camera plus two spectrographs to capture light from celestial objects and an array of silicon chips that convert incoming light into electrical signals.
Magnetic tapes containing information captured by the camera are sent to Fermi National Accelerator Laboratory outside Chicago, where powerful computers sort through the data. Interesting objects are selected for follow-up spectroscopy. Astronomers who find something of interest also can take a closer look at the object using the 3.5-meter telescope at Apache Point. This telescope can be remotely controlled by researchers anywhere in the world.
The information captured in the survey – some 20 to 40 trillion bytes of data – is more than the data generated by the Human Genome Project and enough to fill all the books in the Library of Congress. The full SDSS database of imaging and spectroscopy is being made available to both astronomers and the general public. Public releases of data are now being made about every six months.
Major findings to date
Anderson says the survey has produced several major findings or discoveries to date. For example, by studying the light coming from the distant quasars discovered in the survey, researchers have been able to determine when starlight first appeared in the universe.
“Shortly after the universe was formed it was a cold, dark place,” Anderson explains. “Then some gas clouds collapsed and the first generation of stars was born. Using data from the SDSS, we have been able to pinpoint when this happened – more than 10 billion years ago.”
SDSS data also has helped scientists confirm the existence of “dark energy,” a force of unknown origin that counteracts the effects of gravity and is causing galaxies to accelerate away from each other. Scientists now believe that about 73 percent of the universe is made of this dark energy.
Science magazine called this discovery the top scientific breakthrough in 2003. It has helped scientists pinpoint the age of the universe, now believed to be about 13.7 billion years old, and helped them determine the rate at which the universe is expanding.
Anderson says the project has led to many peripheral discoveries as well. For example, the survey has found large numbers of brown dwarfs, a rare breed of low-mass stars.
“We’ve also found some strange objects that we still can’t identify,” Anderson says.
The project also has opened up new fields of astronomy. Prior to the Sloan Digital Sky Survey, astronomers had found 10,000 quasars. Today, there are 100,000 known quasars and this number is rising. Researchers are now studying these quasars with other telescopes to learn how the chemical composition of the universe has changed over time.
Although the project was not designed to study our own galaxy, Anderson says
researchers have made some new discover-ies about it as well. He hopes that once the SDSS is completed, astronomers will be able to find funding for further studies of the Milky Way. Such studies may help astronomers detect remnants of proto-galaxies that merged together to form our galaxy and to understand how galaxies form.
Several NMSU faculty members and students are using data from the Sloan Digital Sky Survey to conduct their research.
Peregrine McGehee, a student of Anderson’s, has been studying stellar magnetic fields and stellar evolution using SDSS observations. His research centers on how low-mass stars and brown dwarfs are formed. He also has been investigating the distribution of dust clouds within our own Milky Way Galaxy.
Ph.D. student Joe Helmboldt is writing his thesis on elliptical galaxies that are still forming new stars. Elliptical galaxies with ongoing star formation are rare.
Anatoly Klypin, associate professor of astronomy, used the SDSS data on more than 250,000 galaxies to seek evidence of the gravitational effects of dark matter, an exotic form of matter that researchers believe constitutes about 23 percent of the universe and most of its mass.
“The unique capabilities of the SDSS enabled us to probe dark matter in a way that cannot be done by any other current experiment,” Klypkin says.
One of Klypin’s students, Mayrita Vitvitska, is using data from the survey to work on her Ph.D. thesis, which is on the nature of the dark matter “halos” that surround galaxies.
Jon Holtzman, associate professor of astronomy, and Ph.D. student Sarah Bates are using information from the SDSS to try to discover new globular clusters in our neighboring galaxy, the Andromeda Galaxy. Globular clusters are clusters of very old stars that can provide clues about how much mass a galaxy has. Holtzman and Bates discovered their first new globular cluster in February.
“The fact that the SDSS takes photos in five colors enables us to learn more about the age and composition of these star clusters,” Holtzman says. Holtzman also is using the survey to conduct research on the shape of galaxies and what this tells us about how they were formed.
In March, the NMSU researchers and other astronomers working on various aspects of the Sloan Digital Sky Survey gathered at NMSU to compare notes on their research. Nearly 300 major scientific papers have already come out of the project, and about 50 Ph.D. theses based on the project are either completed or under way.
“We’ll be mining this archive of data for years to come,” Anderson says.
For more information on the Sloan Digital Sky Survey, visit the project Web site at www.sdss.org.
A video about the project, titled “Capturing Heaven in a Box – The Sloan Digital Sky Survey,” is available from KRWG-TV, (505) 646-2222.