Press Release

Universe Re-Ionized 900 Million Years After Birth

May 25, 2006

(A summary of basic information from the May 19, 2006, press release from Kyoto University. For details in Japanese please go to:

Subaru observations of the most distant known gamma-ray bust show that most hydrogen between the galaxies had become ionized 900 million years after the Big Bang. This study by researchers from Kyoto University, Tokyo Institute of Technology, and the National Astronomical Observatory of Japan dates the formation of the first generations of stars and galaxies to an ever precise time than previous studies, and demonstrates the power of gamma-ray bursts to illuminate the early universe.

Fourteen billion years ago the universe came into existence as a fiery furnace of energy and matter. As it expanded, it cooled. After 300 thousand years it reached a temperature of about 3000 degrees, allowing atomic nuclei and electrons to combine and form electrically neutral atoms. After that, gravity brought matter together to form the stars and galaxies we know today.

However, we also know from previous studies of quasars, energetic beacons from the early universe, that most of the sparse matter in between galaxies is now ionized - split back into nuclei and elections. How and when this happened is one of the central questions of modern astronomy because the re-ionization of the universe and the early genealogy of stars and galaxies are closely linked. Ultra- violet radiation from the first generations of stars is one of the strongest candidates for cause of the re-ionization
In recent years, astronomers have learned that gamma-ray bursts can be even better beacons than quasars for illuminating the process of the re-ionization of the universe. Quasars shine from the release of gravitational energy as matter falls into a black hole. When observing quasars, it is difficult to disentangle the quasar, the quasar's effect on its immediate environment, and the environment in the vastness between the quasar and Earth. Gamma-ray bursts are now known to originate from extremely energetic versions of a supernova, the death throws of a massive star. These events do not disturb their immediate environment, so extracting information about  the gas between the galaxies is much easier.

On September 4, 2005, the Swift Gamma-Ray Burst Mission detected a gamma-ray burst, GRB 050904, that had occurred 900 million years after the birth of the universe. Japanese researchers using the Subaru telescope were able to catch a high-resolution spectrum of the visible light after-glow of the burst. A detailed analysis of this spectrum has revealed that at the time of the burst, more than 80% of the hydrogen between galaxies was already ionized.

Dr. Tomonori Totani from the team says "our technology to observe gamma-ray bursts is really maturing - allowing us to understand earlier times of the universe with ever greater detail. This event demonstrates the power and potential of studying the early universe using gamma-ray bursts. It opens a new frontier of observational cosmology for astronomers worldwide."

Now the team awaits the detection of an even more distant gamma-ray burst.

These results will be published in the June 25, 2006 edition of the Publications of the Astronomical Society of Japan.



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