Israelis, Japanese discover ancient supernovae

The largest sample ever found of the most distant exploding stars have been discovered using a device called the Subaru Telescope.

By JUDY SIEGEL-ITZKOVICH
supernovae (photo credit: Thinkstock)
supernovae
(photo credit: Thinkstock)
The largest sample ever found of the most distant exploding stars called supernovae have been discovered by a team of Israeli and Japanese astronomers, who used a device called the Subaru Telescope, located at the 4,200 meter-high summit of Mauna Kea on the island of Hawaii.
The telescope was given the Japanese named for the Pleiades star cluster, which inspired the logo of the Japanese makers of one of Israel’s most popular car, Subaru. It alludes to the six companies that merged to create the mega-company that manufactures it. The news was jointly released for publication on Tuesday.
The researchers were Tel Aviv University doctoral student Or Graur of the Department of Astronomy and Astrophysics, who is currently working at the American Museum of Natural History in New York and Prof. Dan Maoz, under whom Graur studies, and Associate Prof. Tomonori Totani of Kyoto University. The results were described in a paper in the new October issue of the Monthly Notices of the Royal Astronomical Society.
The Subaru telescope represents a new generation in telescope design not only because of the size of its primary mirror with an effective aperture of 8.2 meters, but also because of the revolutionary technologies used to achieve outstanding performance. An active support system that maintains an unprecedented high mirror surface accuracy, a new enclosure design to suppress local atmospheric turbulence, an extremely accurate tracking mechanism using magnetic driving systems, seven observational instruments installed at the four foci, and an autoexchanger system to use the observational instruments effectively are just some of the unique features associated with this telescope. These sophisticated systems were used and fine tuned to assemble the largest sample ever found of the most distant exploding stars called supernovae, which emitted their light about 10 billion years ago, long before the Earth was formed.
The Japanese astrophysicist used this sample of ancient supernovae to determine how frequently such explosions of stars occurred in the infant universe. Supernovae, nature’s element factories, have substantial importance in astrophysics. They are composed of all the elements in the periodic table that are heavier than oxygen and were formed through nuclear reactions immediately preceding and during these colossal explosions. These spectacular events fling these elements into interstellar space, where they serve as raw materials for new generations of stars and planets.
Thus, the scientists said, the atoms in our bodies – like the calcium atoms in our bones or the iron atoms in our blood – were created in supernovae. By tracking the frequency and types of supernova explosions back through cosmic time, astronomers can reconstruct the universe’s history of element creation – from the plain mix of hydrogen and helium that existed for the first billion years or so after the Big Bang until the elemental richness evident today.
However, looking back in time requires looking out to great distances, which means that even these bright explosions are exceedingly faint and difficult to spot. To overcome this obstacle, the team took advantage of a combination of the Subaru Telescope’s assets: the huge light-collecting power of its large mirror; the sharpness of its images and the wide field of view of its prime focus camera (Suprime-Cam).
On four separate occasions, they pointed the telescope toward one single field called the Subaru Deep Field, which spans an area of the sky similar to that covered by the full moon and had previously been studied in great detail by Subaru scientists. By “staring” with the telescope at this single field, they let the faint light from the most distant galaxies and supernovae accumulate over several nights at a time, thus forming a very long and deep exposure of the field. Each of the four observations caught about 40 supernovae in the act of exploding among the 150,000 galaxies in the field. Altogether, the team discovered 150 explosions, including a dozen that rank among the most distant and ancient ever seen.
The team’s data analysis showed that supernovae of the so-called “thermonuclear” type were exploding about five times more frequently in the young universe, about 10 billion years ago than they do today, the team said. Thermonuclear supernovae, often called Type-Ia supernovae, are one of the main sources of the element iron in the universe. These explosions have served as cosmic distance markers for astronomers.
Over the past decade, they have revealed that the universal expansion, in which all galaxies are receding from each other, is actually accelerating under the influence of mysterious dark energy. However, the nature of the thermonuclear supernovae themselves is poorly understood, and there has been fierce debate about the identity of the pre-explosion stars or stellar systems.
By revealing the range of the ages of the stars that explode in this way, the team’s new findings provide some important clues to solving this mystery. The results correspond closely to a scenario in which a thermonuclear supernovae is the outcome of the merger of a pair of compact stellar remnants called white dwarfs. Future observations with the next-generation Subaru imaging camera, Hyper Suprime-Cam, will permit the discovery of even larger and more distant supernova samples, and allow for further testing of this conclusion.