Research @ KICP
January 15, 2006
SDSS-II Supernova Survey explodes with new findings
by Joshua A. Frieman
''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 Astrophysical Research Consortium's 3.5-meter telescope, the Hobby- Eberly 9.2-meter telescope in Texas, the William Herschel 4.2-meter telescope in the Canary Islands, Japan's Subaru 8.2-meter telescope, Keck 10-meter telescope in Hawaii and the MDM Hiltner 2.4-meter telescope in Arizona.
''Follow up observations that measure the spectrum of the supernova are critical,'' explained team member John Marriner of Fermilab. ''First, they confirm that the candidate is a Type Ia rather than some other kind of supernova, and second, they determine the velocity at which it is receding from the Earth.'' That recession velocity is caused by 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 cluster of 10 dual-processor computers at the observatory automatically processes the data and looks for differences between the images just taken and those of the same part of the sky in previous years.
''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 Kavli Institute for Cosmological Physics at the University of Chicago. ''This accumulation of 70 gigabytes of images each night is equal to more than 100 CDs or 15 DVDs.''
''All of the well-known results on the accelerating universe have so far come from comparing nearby supernovae to distant supernovae that were discovered and measured in different kinds of surveys,'' added Frieman. ''The SDSS-II survey fills in the missing rungs on the ladder.''
The race to improve the quality of supernova samples over a range of distances is heating up.
''The Sloan 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.'' (ESSENCE is a five-year supernova survey designed to constrain the physics of Dark Energy.)
The SDSS researchers are also looking forward to some surprises in the ''astronomical zoo.''
''We focused on the type Ia supernovae this season, because of the exciting cosmology applications, but the SDSS gives us a unique tool to study unusual supernova types,'' noted Stanford University's Roger Romani. ''We have shown that we can sift through hundreds of new supernovae to select the rare gems. With a survey volume this large, these rarities turn up frequently enough to allow serious investigation of their place in the astronomical zoo.''
Type Ia Supernovae are formed when white dwarf stars - the remnants of stars similar to our Sun -- collapse inward and blow up like an atomic bomb in brief but intense bursts of energy. The white dwarf accumulates gas from a companion star until it explodes, spewing gas and particles of iron, nickel and cobalt. The brightness of the Type Ia light peaks about three weeks after the explosion and declines over a period of months.
Information about the newly discovered SDSS-II supernovae has been distributed to the astronomical community via the International Astronomical Union circulars and the Central Bureau for Electronic Telegrams. These listings of supernova discovery dates, positions, magnitudes, and list of co-authors helps the community perform follow- up research.
The Sloan Supernova Survey is one of three research and discovery components of The Sloan Digital Sky Survey (II), which will run through mid-2008.
ABOUT THE SLOAN DIGITAL SKY SURVEY (SDSS-II)
The Sloan Digital Sky Survey is the most ambitious astronomical survey project ever undertaken. The survey will map one- quarter of the entire sky, determining the positions and absolute brightnesses of more than 100 million celestial objects. It will also measure the distances to more than a million galaxies and quasars from the Apache Point Observatory in New Mexico.
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.