Two 'super-luminous' supernovae -- stellar explosions 10-100 times brighter than other supernova types -- have been detected in the distant universe, using the W. M. Keck Observatory on the top Mauna Kea, Hawaii. The discovery, reported online in Nature today [http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11521.html], sets a record for the most distant supernova yet detected, and offers the rare possibility of observing the explosions of the first stars to form after the Big Bang.
"The type of supernovae we've found are extremely rare," said Jeff Cooke, astrophysicist at Swinburne University of Technology, whose team made the discovery. "In fact, only one has been discovered prior to our work. This particular type of supernova results from the death of a very massive star (about 100 to 250 times the mass of our Sun) and explodes in a completely different way compared to other supernovae. Discovering and studying these events provides us with observational examples to better understand them and the chemicals they eject into the universe when they die."
Super-luminous supernovae were discovered only a few years ago, and are rare in the nearby universe. Their origins are not well understood, but a small subset of them is thought to occur when extremely massive stars undergo a nuclear explosion triggered by the conversion of photons into electron-positron pairs. Such events are expected to have occurred more frequently in the early universe (at high redshift), when massive stars were more common. This, and the extreme brightness of these events, encouraged Cooke and colleagues to search for super-luminous supernovae at redshifts, z, greater than 2, when the universe was less than one-quarter of its present age.
"We used LRIS (Low Resolution Imaging Spectrometer) on Keck I to get the deep spectroscopy to confirm the host redshifts and to search for late-time emission from the supernovae," Cooke said. "The initial detections were found in the CFHT Legacy Survey Deep fields. The light from the supernovae arrived here on Earth 4 to 6 years ago. To confirm their distances, we need to get a spectrum of their host galaxies which are very faint because of their extreme distance. The large aperture of Keck and the high sensitivity of LRIS made this possible. In addition, some supernovae have bright enough emission features that persist for years after they explode. The deep Keck spectroscopy is able to detect these lines as a further means of confirmation and study."
Cooke and co-workers searched through a large volume of the universe at z greater than or equal to 2, and found two super-luminous supernovae, at redshifts of 2.05 and 3.90 -- breaking the previous supernova redshift record of 2.36, and implying a production rate of super-luminous supernovae at these redshifts at least 10 times higher than in the nearby universe. Although the spectra of these two objects make it unlikely that their progenitors were among the first generation of stars, the present results suggest that detection of those stars may not be far from our grasp.
Detecting the first stars allows us much greater understanding of the first stars in the universe, Cooke said. "Shortly after the Big Bang, there was only hydrogen and helium in the universe," he said. "All the other elements that we see around us today, such as carbon, oxygen, iron, and silicon, were manufactured in the cores of stars or during supernova explosions. The first stars to form after the Big Bang laid the framework for the long process of enriching the universe that eventually produced the diverse set of galaxies, stars, and planets we see around us today. Our discoveries probe an early time in the universe that overlaps with the time we expect to see the first stars."
Communications Officer, Advancement
W. M. Keck Observatory
Swinburne University of Technology, Hawthorn, Australia
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Caption: High-resolution simulation of a galaxy hosting a super-luminous supernova and its chaotic environment in the early universe.
Credit: Adrian Malec and Marie Martig (Swinburne University)
The W. M. Keck Observatory operates two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.