Cosmic blast sets distance mark

[HURAKAN]

Cosmic blast sets distance mark
By Jonathan Amos
Science reporter, BBC News

The cataclysmic explosion of a giant star early in the history of the Universe is the most distant single object ever detected by telescopes.

The colossal blast was picked up first by Nasa's Swift space observatory which is tuned to see the high-energy gamma-rays emitted from extreme events.

Other telescopes then followed up the signal, confirming the source to be more 13 billion light-years away.

Scientists say the star's destruction probably resulted in a black hole.

"This gets us into a realm where we've never been before," said Nial Tanvir, of the University of Leicester, UK.

"This is the most remote gamma-ray burst (GRB) ever detected, and also the most distant object ever discovered."

Fast mover

The Swift satellite was launched in 2004 to investigate the energetic flashes that characterise some of the Universe's most violent happenings.
A GAMMA-RAY BURST RECIPE
# Models assume GRBs arise when giant stars burn out and collapse
# During collapse, super-fast jets of matter burst out from the stars
# Collisions occur with gas already shed by the dying behemoths
# The interaction generates the energetic signals detected by Swift
# Remnants of the huge stars end their days as black holes

Led by the US space agency, the mission has significant UK and Italian contributions.

It is a three-in-one observatory. Its Burst Alert Telescope is set up to catch the initial flood of gamma rays. The spacecraft then swings itself to look directly into the flash with X-ray and ultraviolet/visible telescopes.

This longer wavelength afterglow lasts on the order of hours to days and Swift also calls up ground-based observatories to join the spectacle. Indeed, it is the ground campaign that establishes the distance.

This burst, dubbed GRB 090423, was detected by Swift on 23 April.

Follow-up observations were led by the United Kingdom Infrared Telescope and the Gemini North Telescope, both on Mauna Kea, Hawaii.

Cosmic expansion

Analysis of the light spectrum confirmed the blast had a redshift of 8.2. Redshift is a measure of the degree to which light has been "stretched" by the expansion of the Universe. The greater the redshift, the more distant the object and the earlier it is being seen in cosmic history.

The figure 8.2 equates to a distance of 13.035 billion light-years. Put another way, the explosion is being viewed when the Universe was only 630 million years old, a mere one-20th of its current age (estimated to be 13.7 billion years old).

The previous record holder was a GRB witnessed, also by Swift, in September 2008. It had a redshift of 6.7, making it 190 million light-years closer than GRB 090423.

Scientists have seen what they believe may be faint galaxies at redshifts 8-10, but their true nature is still being investigated.

Researchers are very keen to probe these great distances because they will learn how the early Universe evolved, and that will help them explain why the cosmos looks like it does now.

'Fried' gas

Scientists believe the super-hot conditions that existed after the Big Bang eventually cooled sufficiently to allow protons, neutrons and electrons to form neutral atoms of hydrogen and helium.

When they did, from infalling clouds of hydrogen and helium, the "dark ages" gave way to what has been dubbed the "cosmic renaissance".

What is more, these hot, young stars produced intense ultraviolet radiation which "fried" the gas in the Universe - to produce the diffuse intergalactic plasma detectable today.

The star which produced GRB 090423 was very probably among that population of giants responsible for this re-ionisation.

"We're now starting to see some of the very earliest stars," said Dr Tanvir.

"The re-ionisation era was a change that occurred in the Universe when the first stars switched on; but the actual timescale for that and the processes at work are poorly understood because we have so few observations. We're now pushing into that epoch."

Jonathan.Amos-INTERNET@bbc.co.uk
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