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Gamma-ray burst is most distant yet seen

grbmedium.jpgA record has been set for the most distant gamma-ray burst, astronomers announced at the American Astronomical Society meeting in Boston today. The result pushes back the time by which stars and galaxies must have formed to just 500 million years after the Big Bang, 13.7 billion years ago.

“This tells us there was already huge star formation activity going on,” says astronomer Antonio Cucchiara of Lawrence Berkeley National Laboratory in California, who is first author on a paper about the result that is due to be posted on the arxiv preprint archive later today.

The brightest objects in the universe, gamma ray bursts (GRBs), are thought to caused by massive stars going supernovae and collapsing to form black holes or neutron stars. The GRB discussed today at the meeting, GRB090329B, was first picked up in 2009 by NASA’s Swift satellite, and then imaged by the 8.1 meter Gemini North telescope in Mauna Kea, Hawaii. But due to bad weather in Hawaii, the research team was not able to study the afterglow long enough to obtain a detailed spectrum that would have pinpointed how faraway it was.


Instead, the team compared the GRB’s dimness in visible light to its relative brightness in the infrared, which provides a potential measure of how much absorbing hydrogen gas the light traveled through on its way to Earth and hence how faraway the burst is. They concluded the burst was either at a redshift of 9.4 –setting the record for a detected GRB’s remoteness from Earth – or in a much more nearby galaxy shrouded in dust. Later the team imaged the region of the sky where the GRB occurred using the Hubble Space Telescope and saw no sign of a nearby galaxy, ruling out that explanation, says Cucchiara.

“I’m very impressed with this team. They worked diligently to really understand what we can say about the redshift of this burst and it looks very solid to me,” says Neil Gehrels, principal investigator of Swift and an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

The previous record-holder for gamma ray bursts was GRB090423, which had a redshift of 8.2, reported in 2009. Earlier this year, Rychard Bouwens, now at Leiden University in the Netherlands, and his colleagues described detection of a galaxy at redshift 10, although their distance estimate is less precise than that reported for GRB 090329B. Andrew Levan of the University of Warwick in the UK, part of the team behind today’s announcement, says observing galaxies and detecting gamma ray bursts provide complementary ways to learn about the early universe, and that the advantage of gamma ray bursts is that they come about through a specific process that can be compared at early and late times to get an idea how stars have changed throughout cosmic history.

For example, the gamma ray flash from GRB 090329B lasted just five seconds, suggesting it was not from the first generation of stars to form after the Big Bang, which are hypothesized to be more massive stars with brighter, longer-lasting emissions, Levan says. Measurements of the cosmic microwave background suggest the first generation of stars might appear around a redshift 10, however. “We’re probing into the region where you would expect to see the first stars,” he says.

Three planned future missions, Lobster, the Joint Astrophysics Nascent Universe Satellite, and the French-Chinese Space-based Multi-band Astronomical Variable Object Monitor, are expected to focus on collecting more, high-redshift gamma ray bursts.

Image: Gemini Observatory / AURA / Levan, Tanvir, Cucchiara

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