Most distant object in the universe spotted

by Rachel Courtland at NewScientist.com

Astronomers have spotted the most distant object yet confirmed in the universe – a self-destructing star that exploded 13.1 billion light years from Earth. It detonated just 640 million years after the big bang, around the end of the cosmic "dark ages", when the first stars and galaxies were lighting up space.

The object is a gamma-ray burst (GRB) – the brightest type of stellar explosion. GRBs occur when massive, spinning stars collapse to form black holes and spew out jets of gas at nearly the speed of light. These jets send gamma rays our way, along with "afterglows" at other wavelengths, which are produced when the jet heats up surrounding gas.

The burst, dubbed GRB 090423 for the date of its discovery last Thursday, was originally spotted by NASA's Swift satellite at 0755 GMT.

Within an hour, astronomers began training ground-based telescopes on the same patch of sky to study the burst's infrared afterglow. Some of the first observations were made on Mauna Kea in Hawaii with the United Kingdom Infrared Telescope and the Gemini North telescope.

Other telescopes later measured the spectrum of the afterglow, revealing that the burst detonated about 13.1 billion light years from Earth. "It's the most distance gamma-ray burst, but it's also the most distant object in the universe overall," says Edo Berger of the Harvard-Smithsonian Center for Astrophysics, a member of the team that observed the afterglow with Gemini North.
Stretched light

To gauge an object's distance, astronomers measure how much an object's light has been stretched, or reddened, by the expansion of space. This burst lies at a redshift of 8.2, more distant than the previous GRB record holder, which lay at a redshift of 6.7.

Other astronomers have claimed to find galaxies at even greater distances – at redshifts of 10 and 9, but those findings are still ambiguous, says Joshua Bloom of the University of California, Berkeley, who observed the afterglow using the Gemini South telescope in Chile. Until now, the record holder for the farthest galaxy had a spectroscopically confirmed redshift of 6.96.

The burst's immense distance makes the now-dead star the earliest object to be discovered from an era called 'reionisation', which occurred within the first billion years after the big bang. At that time, an obscuring fog of neutral hydrogen atoms was being burned off by radiation from the first stars and galaxies, and possibly also from the annihilation of dark matter particles.
'Watershed event'

"For astronomy, this is a watershed event," Bloom told New Scientist."This is the beginning of the study of the universe as it was before most of the structure that we know about today came into being."

The timing of the period of reionisation is still unclear, Bloom says. If astronomers can find more gamma-ray bursts at even greater distances, they could use their spectra to determine how quickly the universe became transparent and what was responsible for the process.

"In principle, you can see very early times in the universe [with GRBs], when everything else was too faint," says Nial Tanvir of the University of Leicester in the UK, a member of a team that used the Very Large Telescope in Chile to make one of the first measurements of the distance of the burst.

Distant blasts could also help pinpoint the locations of faint GRB host galaxies that could be detected by space telescopes like the soon-to-be-refurbished Hubble Space Telescope or NASA's infrared James Webb Telescope, which is set to launch in 2013.
Sensitive and fast

But building up a picture of the early universe will require finding many more distant bursts, and progress in discovering distant bursts has been slow. Swift has found 120 bursts with measured distances, but only three – including this one – date from the first billion years of the universe's history.

That is in part because stars did not form at high rates in the very early universe, before a redshift of about 5, and so they did not explode often as GRBs.

But it is also because infrared detectors that are both sensitive and quick enough to measure very distant, short-lived GRB afterglows have only recently begun operating. As a result, astronomers may have missed out on identifying some of the most distant GRBs identified by Swift.

Berger hopes the discovery of this object will hasten the development of new telescopes that could discover such afterglows with even greater efficiency.

"As a single object, [the burst] is an amazing proof of concept," says Berger. "I think we've shown that's a worthwhile investment because [distant bursts] actually do exist."

NASA is considering one such telescope, called the Joint Astrophysics Nascent Universe Satellite (JANUS), for funding this year