A faint gamma-ray burst (GRB) captured last Thursday by NASA’s Swift satellite has smashed the record for the earliest, most-distant known object in the universe — with a redshift of about 8.2.
The burst, named GRB 090423 for its discovery date, went off in Leo and was seen to last for 10 seconds. Several teams, including a group using the Gemini-North telescope in Hawaii and a European group using the Very Large Telescope in Chile, followed up the Swift detection by observing the burst’s fading infrared afterglow. Based on how much the afterglow’s light was redshifted (stretched) by cosmic expansion since the era when the burst happened, the group determined that it went off about 630 million years after the Big Bang.
This means that the GRB's gamma rays traveled for a mind-boggling 13.1 billion years before reaching Earth. That's so far back in time that it's meaningless to assign a specific "distance," since large distances in the universe have themselves expanded by a factor of 9.2 since that time. From the burst's perspective, Earth's formation lay 8.5 billion years in the future.
The redshift of 8.2 also means that the observed duration of the burst was stretched out by 9.2 times. In other words, what Swift saw lasting for 10 seconds actually took just over 1 second to happen in its own "rest frame." But this does not necessarily mean that GRB 090423 was a "short burst," a type of event that seems to have a different cause from the usual "long bursts." It could have emitted gamma rays for much longer than 1 second but with only a brief peak that was strong enough for Swift to see.
"This is a fantastic burst," says Swift lead scientist Neil Gehrels (NASA/Goddard Space Flight Center). "We have been waiting for 4.5 years since the Swift launch for such a distant GRB."
"This burst breaks all the records for galaxies, quasars, anything," adds Edo Berger (Harvard-Smithsonian Center for Astrophysics), who led the Gemini observations. "This is more than just a breaking a record. This is a demonstration that the best way to study the young universe and the first generation of stars and galaxies is to use GRBs."
A GRB comes from the cataclysmic explosion of a massive star. Since the Swift satellite was launched in 2004, it has undoubtedly seen GRBs with even higher redshifts, but most bursts at this immense distance have afterglows so faint that astronomers are unable to see the afterglows, which are necessary for measuring a redshift. Nevertheless, this detection demonstrates that until NASA launches the James Webb Space Telescope next decade, GRBs offer the most promising method for probing the universe at redshifts of 8 and higher.
The discovery also proves massive stars capable of exploding as GRBs existed when the universe was only 630 million years old. This is not a surprise; cosmologists think the very first stars formed when the universe was between 200 and 400 million years old — that is, between redshifts 12 and 20.