By Ron Cowen
LONG BEACH, Calif. — Like a searchlight illuminating the distant past, the afterglow of a powerful gamma-ray burst has revealed what a stellar nursery in a remote galaxy looked like just 3 billion years after the Big Bang. The burst offers one of the earliest views of a star-forming region in the universe, which is now 13.7 billion years old.
The gamma-ray burst, recorded on June 7, 2008, and dubbed GRB 080607, is believed to have been generated when a massive star suddenly collapsed to form a black hole. While the burst itself lasted for only seconds, its fading afterglow in visible light remained remarkably bright for a full hour.
Jason Prochaska of the University of California, Santa Cruz, and his colleagues began observing the visible-light afterglow with the Keck I Telescope on Hawaii’s Mauna Kea less than 20 minutes after the burst was recorded by NASA’s orbiting Swift observatory.
Prochaska reported the findings on January 6 at the winter meeting of the American Astronomical Society and his team will also describe the study in an upcoming Astrophysical Journal Letters.
The Keck spectrum of the afterglow revealed that the burst originated in a galaxy so remote that the light now reaching Earth was emitted when the galaxy was only 3 billion years old. Among thousands of quasars, hundreds of stars and several tens of gamma-ray bursts Prochaska has examined, “this is the most exciting spectrum I have ever studied,” he says. The spectrum provides “the first view of a star-forming region with a gamma-ray burst,” revealing details on spatial scales of just a few light-years, much finer than can be seen by directly imaging a distant galaxy in visible light or radio.
The galaxy shows a remarkably similar enrichment in chemical elements heavier than helium, along with dust and molecular cloud properties, to what is observed in the Milky Way today. “This is really our first view of these properties in such a distant galaxy, and the surprise is really to see such a mature galaxy in our distant past,” Prochaska says.
The finding “demonstrates the ability of gamma-ray bursts through their brilliance to illuminate the properties of the [distant] universe,” comments theorist Don Lamb of the University of Chicago. “They have the power to make it possible to measure things that are otherwise unobservable — in this case the properties of a cold, dark and dense molecular cloud as it was 10 billion years ago.”
The forensic evidence found by Prochaska and his collaborators “points more strongly than ever before to dense molecular clouds as the scene of the deaths of the massive stars that produce gamma-ray bursts,” adds Lamb.
“Within the spectrum, there are several tens of absorption features which remain unidentified,” he adds. “In comparison, there may be only a handful of absorption lines, if any, that I would say are unidentified in the hundreds of other spectra that I have examined closely. Odds are, we are seeing these [absorption lines] for the first time on Earth.”
The afterglow had to be unusually bright to reveal so much about the dusty, star-forming region of the host galaxy from which it originated. Prochaska estimates that the event is the second most luminous afterglow on record, and for an hour remained 10,000 times more luminous than a typical quasar. Had the afterglow been much dimmer, then dust in the galaxy — which absorbs 99 percent of visible light — would have rendered the afterglow invisible.
Over the past several years, astronomers have detected gamma-ray bursts even more remote than this one, bursts that reveal the existence of massive stars as early as 1 billion years after the Big Bang. But because the spectra of the afterglow from GRB 080607 has a much stronger signal and covers a much wider range of wavelengths “we learn far more about the nature of galaxies” from long ago, Prochaska says.
