July 8, 2009
Astronomers have again rewritten the record books for discovering the most distant supernovae. Using Hawaii's W. M. Keck Observatory and Canada-France-Hawaii Telescope (CFHT), a team has identified remnants of two massive stars that exploded roughly 11 billion years ago.
Studying the deaths of these early stars is essential to understanding the evolution of the universe and how its elements were formed and distributed to create later stars and even planets, said Jeff Cooke of the University of California at Irvine.
He added that while the newly identified explosions may be the farthest of any supernovae type found to date, the innovative method developed to identify the explosions should make it possible to discover even more distant supernovae — possibly even a few of the very first stars to blow apart.
Cooke developed this new method to study the explosive death of stars that are 50 to 100 times the mass of the Sun. The progenitor stars of this kind of supernovae, type IIn, are distinct because they shed most of their material into the cosmos just before they die. When the stars finally explode, they spew out their remaining material, which plows into the previously expelled gas. The collisions make the entire stellar remnant so bright that its glow can still be detected many years after the star's demise.
To find the most distant of these supernovae, the astronomers examined archival data from the CFHT Legacy Survey to identify four, extremely distant objects that appeared to brighten and then fade over time, resembling distant supernovae. Cooke explained that cosmologists typically identify supernovae by comparing nightly images of the same patch of space taken at regular intervals throughout the year. The images show several hundred to thousands of galaxies. A slight increase in the amount of light in any one of the galaxies in one image compared to the previous image may indicate a star has blown apart and died.
Using this knowledge, the astronomers stacked and blended a year's worth of CFHT images taken of the same, dark patch of sky and did this for 4 separate years. Stacking the images into one composite enable the team to detect fainter objects and thereby probe farther back in the universe. "It's like in photography when you open the shutter for a long time. You'll collect more light with a longer exposure," Cooke said.
By comparing composite images over the 4 years, Cooke's team identified four potential supernovae. The astronomers then used the Low Resolution Imaging Spectrograph (LRIS) on the Keck I telescope and the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope to analyze the spectrum of light that each object emitted to determine the objects' composition and distance. The data showed that the light from the supernovae had traveled nearly 11 billion light years to reach Earth.
Cooke's technique is "powerful and reliable" because "it's simple, clean, and the results are unambiguous. In retrospect, I can't believe we haven't capitalized on this method sooner," said Alicia Soderberg who studies supernovae at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, but was not involved in the study.
The technique will revitalize research on this kind of supernovae and will provide astronomers with a much-needed process to probe the deaths of some of the earliest stars in the universe, she added.
Prior to this discovery, astronomers' records showed that the most distant supernova of this type exploded roughly 6 billion years ago, and the most distant of any supernovae type exploded roughly 9 billion years ago. Cooke said that by studying extremely distant supernovae, astronomers would better understand where stars were exploding just after the Big Bang and how stellar properties change as the universe evolved. And, because stars form heavier and heavier elements in their core, the technique might also give astronomers a glimpse of how the elements essential to planet formation and to the existence of life were initially created and distributed throughout the cosmos.
"This new method could not have been published at a better time," Soderberg said, explaining that many large survey telescopes, such as the Large Synoptic Survey Telescope, will soon be online to identify thousands of candidate supernovae. Astronomers can then use large eight to ten meter telescopes, such as Keck, to obtain the necessary deep spectra of the supernovae to determine their distance and the abundance of elements that they spew into space after they explode.