University Park, Pa. -- Scientists from the Sloan Digital Sky Survey (SDSS), including a Penn State astrophysicist, described on Thursday, Jan. 12, the first results of their search for distant supernovae -- exploding stars located in distant galaxies that are billions of times more luminous than the sun. The study has discovered more than 100 supernovae in the first 90 days of the program.
Donald Schneider, Penn State professor of astronomy and astrophysics, is a member of the SDSS Supernovae team. Schneider also is the chairman of the SDSS Quasar Science Group and the SDSS scientific publications coordinator.
"The supernovae, which were found in galaxies billions of light years away, are one of our most sensitive probes of the evolution of the universe," Schneider said. "Candidate supernovae were identified from images taken with the SDSS telescope in New Mexico. Several telescopes around the world, including the Hobby-Eberly Telescope, obtained spectra of the candidates to determine whether the objects were indeed supernovae."
The Hobby-Eberly Telescope is located at the McDonald Observatory in Texas and is operated by Penn State, the University of Texas, Stanford University, the University of Goettingen in Germany and the University of Munich in Germany.
The unprecedented number of new "Type-Ia" supernovae discoveries, precisely 139 of them, were logged in just 90 days by the Sloan Digital Sky Survey's SDSS-II Supernova Survey during its first campaign last fall.
"Finding so many supernovae in such a short time is unprecedented because the SDSS probes a larger volume of space than previous surveys," explained supernova team co-leader Joshua Frieman of the Fermi National Accelerator Laboratory (Fermilab) and the University of Chicago. The discoveries were made public at the American Astronomical Society's winter meeting in Washington, D.C.
These supernovae will be used to provide more precise constraints on the nature of the mysterious Dark Energy that is causing the expansion of the universe to speed up. They also will yield greater understanding of supernovae as standard distance signposts in the universe, which astronomers also call "standard candles" or "standard light bulbs."
To find the supernovae, the team used the dedicated SDSS-II 2.5-meter telescope at the Apache Point Observatory in New Mexico. Its 120-megapixel CCD camera is used to scan the same part of the sky roughly every other night over a three-month period, searching for supernovae that explode in any of the 3 million galaxies it sees. Shortly after a supernova explodes, it becomes as bright as an entire galaxy, so it can be seen across vast cosmic distances.
Comparing images taken on different nights, astronomers can find potential supernovae by looking for objects that have brightened over time.
"An advantage of the SDSS is that it carries out nearly simultaneous imaging in five different portions of the optical spectrum, providing measurements of the colors as well as the brightness of objects," explained Masao Sako of Stanford University. Using these colors, Sako and other members of the SDSS-II supernova team were able to zero-in with very high efficiency on the prized Type-Ia supernovae, the most precise standard candles.
The most promising Type-Ia candidates were targeted for follow-up spectroscopy on a variety of other telescopes, including the Hobby-Eberly Telescope's 9.2-meter telescope in Texas, the Astrophysical Research Consortium's 3.5-meter telescope and NMSU's 1-meter telescope at Apache Point, the William Herschel 4.2-meter telescope in the Canary Islands, Japan's Subaru 8-meter telescope and the Keck 10-meter telescope in Hawaii, and the MDM Hiltner 2.4-meter telescope in Arizona.
Follow-up observations that measure the spectrum of a supernova are critical for confirming that the candidate is a Type-Ia rather than some other kind of supernova, and for determining the velocity at which the supernova is receding from the Earth as a result of the expansion of the universe. By combining the brightnesses and recession velocities of large numbers of supernovae, astronomers can unravel the history of the cosmic expansion rate.
Since supernovae are bright only for a few weeks, the candidates must be identified quickly so that their spectra can be measured before they fade.
"A full night of data collection with the telescope yields about 2,400 images, each one roughly equivalent to the image in a four-megapixel digital camera. We process these images with our computers in about 20 hours," explained supernova researcher Richard Kessler of the University of Chicago. "This accumulation of 70 gigabytes of images each night is equal to more than 100 CDs or 15 DVDs."
The new supernova sample bridges the gap between the nearby supernovae found in local surveys and the very distant objects found by deeper surveys of much smaller areas. Previously, only a handful of supernovae at these distances -- about 1 to 3 billion light years from Earth -- had been found, and the race to improve the quality of supernova samples over a range of distances now is heating up.
"The Supernova Legacy Survey and the ESSENCE survey are compiling more distant samples that will include hundreds of supernovae by the time they each finish a few years from now," noted Craig Hogan of the University of Washington. "Combining the SDSS results with these deeper surveys will probe the cosmic expansion and the nature of the dark energy with greater precision."
The Supernova Survey is one of three research and discovery components of The Sloan Digital Sky Survey (II), which will run through mid-2008. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England.