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Swinburne team on Keck discovers farthest supernova ever

The discovery offers the rare possibility of observing the explosions of the first stars to form after the Big Bang.
Supernovae_illustration
High-resolution simulation of a galaxy hosting a superluminous supernova and its chaotic environment in the early universe. // Credit: Adrian Malec and Marie Martig (Swinburne Univ.)
Two superluminous supernovae — stellar explosions 10–100 times brighter than other supernova types — have been detected in the distant universe, using the W. M. Keck Observatory atop Mauna Kea, Hawaii. The discovery sets a record for the most distant supernova yet detected, and it 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 from Swinburne University of Technology in Melbourne, Australia, 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 provide us with observational examples to better understand them and the chemicals they eject into the universe when they die.”

Superluminous 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 superluminous supernovae at redshifts greater than 2, when the universe was less than one-quarter of its present age.

“We used Low Resolution Imaging Spectrometer (LRIS) 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 four to six 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 redshift greater than or equal to 2, and found two superluminous supernovae at redshifts of 2.05 and 3.90 — breaking the previous supernova redshift record of 2.36 and implying a production rate of superluminous 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. 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.”

Two superluminous supernovae — stellar explosions 10–100 times brighter than other supernova types — have been detected in the distant universe, using the W. M. Keck Observatory atop Mauna Kea, Hawaii. The discovery sets a record for the most distant supernova yet detected, and it 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 from Swinburne University of Technology in Melbourne, Australia, 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 provide us with observational examples to better understand them and the chemicals they eject into the universe when they die.”

Superluminous 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 superluminous supernovae at redshifts greater than 2, when the universe was less than one-quarter of its present age.

“We used Low Resolution Imaging Spectrometer (LRIS) 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 four to six 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 redshift greater than or equal to 2, and found two superluminous supernovae at redshifts of 2.05 and 3.90 — breaking the previous supernova redshift record of 2.36 and implying a production rate of superluminous 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. 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.”

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