A team of Japanese, Israeli, and U.S. astronomers used the Subaru Telescope to assemble the largest sample ever found of the most distant exploding stars called supernovae, which emitted their light about ten billion years ago, long before the Earth was formed. The researchers used this sample of ancient supernovae to determine how frequently such explosions of stars occurred in the young universe.

Supernovae have substantial importance in astrophysics. They are nature's element factories: essentially all of the elements in the periodic table that are heavier than oxygen were formed through nuclear reactions immediately preceding and during these colossal explosions. The explosions fling these elements into interstellar space, where they serve as raw materials for new generations of stars and planets.

Thus, 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, up to the elemental richness we see 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 8.2 meter primary 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 analysis of the data showed that supernovae of the so-called "thermonuclear" type were exploding about five times more frequently in the young universe, about ten billion years ago, than they do today. Thermonuclear supernovae, often called Type-Ia supernovae, are one of the main sources of the element iron in the universe.

Equally important, these explosions have served as cosmic distance markers for astronomers. Over the past decade, they have revealed that the expansion of the universe, 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.

The results are described in a paper by Graur et al. in the October 2011 issue of the Monthly Notices of the Royal Astronomical Society. The title is "Supernovae in the Subaru Deep Field: the rate and delay-time distribution of type Ia supernovae out to redshift 2".

Team members
O. Graur (Tel-Aviv University,Israel)

D. Poznanski (LBNL, UC Berkeley, USA; Tel-Aviv University, Israel)

D. Maoz (Tel-Aviv University,Israel)

N. Yasuda (University of Tokyo, Japan)

T. Totani (Kyoto University, Japan)

M. Fukugita (University of Tokyo, Japan)

A. V. Filippenko (UC Berkeley, USA)

R. J. Foley (Harvard/Smithsonian Center for Astrophysics, USA)

J. M. Silverman (UC Berkeley, USA)

A. Gal-Yam (Weizmann Institute of Science, Israel)

A. Horesh (Tel-Aviv University, Israel; Caltech, USA)

B. T. Jannuzi (National Optical Astronomy Observatory, USA)