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Berkeley Astronomers Lift Shroud on Dark Gamma Ray Bursts

Berkeley Astronomers Lift Shroud on Dark Gamma Ray Bursts

Credit: Credit: NASA/SkyWorks Digital

This image represents an artist's conception of a gamma-ray burst destroying a star.

PASADENA, Calif.—Astronomers using the Keck telescopes may have solved the mystery of dark gamma ray bursts—intense flashes of X-ray and gamma-ray radiation that have little to no optical signature. The observations have allowed the astronomers to peer through celestial gas and dust to reveal star formation and stellar death in the dusty corners of otherwise dust-free galaxies.

“We have compelling evidence that a large percentage of star formation in the early Universe is actually hidden by dust, even inside galaxies that do not appear dusty,” astronomer Daniel Perley of the University of California, Berkeley announced June 8, 2009 at the 214th American Astronomical Society meeting in Pasadena, Calif.

Long-duration gamma-ray bursts, the most brilliant flashes of light in the Universe, are thought to originate from the explosion of massive stars. These events create two pencil-like beams of light, akin to lighthouse beacons, bright enough to be seen from as far away as 13 billion light years, which is near the edge of the observable Universe. Most gamma-ray bursts continue to shine brightly in optical light for many hours after the gamma-ray emission subsides, a phenomenon known as an ‘optical afterglow’. Yet, events with little or no detectable visible light, dubbed “dark GRBs,” have puzzled astronomers.

Some have suggested that these GRBs are so far away, and thus at such high redshift, that their optical afterglow is shifted out of the visible wavelengths and into the infrared. The new observations, however, support one other prevailing hypothesis for the existence of dark gamma ray bursts— that dust obscures the visible wavelengths of the gamma ray bursts in galaxies at less extreme distances (less than about 12.9 billion light years from Earth).

“The Perley and Bloom work is a very significant result,” said Wendy Freedman, director of the Carnegie Observatories in Pasadena, Calif. who was not involved in the research. She explained that GRBs have excited the astronomical community since their detection in the 1960s. This study is “fundamental,” she explained, because it provides information about the nature of GRBs in the early Universe. It also provides important information about the formation rate of stars when the Universe was merely 700 million years old, she added.

To draw these conclusions, the team first used the 60-inch Palomar telescope to conduct follow-up observations of 29 bursts discovered by NASA’s Swift gamma-ray satellite, 14 of which were classified as dark. The astronomers then used the Keck I telescope to look for the host galaxies of “dark” GRBs. For 11 of these 14 dark bursts, the team successfully detected a distant galaxy hosting the explosion. The remaining three bursts had faint optical counterparts.

Perley said the results indicate that none of these bursts had come from the most distant regions of the Universe since at distances greater than 12.9 billion light years the optical light would be shifted into the infrared due to the expansion of the Universe. The astronomers’ sample lacks these very high redshift events, which indicates that extremely distant explosions cannot comprise more than a few percent of all gamma-ray bursts, Perley said.

Still, such distant bursts are known to exist. Two months ago a gamma ray burst at a distance of 13.1 billion light years was discovered. Combining information from this event with the rest of the sample, the team now estimates that the fraction of high redshift GRBs is between 0.2 and seven percent.

In this study, because dim optical signatures where identified and none of the 14 bursts in the survey appear to be at a distance of more than 13 billion light years, the astronomers can conclude that the optical dimness of the bursts is due to dust inside the host galaxy. The dust absorbs light from the afterglow before it escapes. But, the starlight shows no recognizable dust signatures, which indicates that the dust may be clumped in patches or clouds where it is difficult to detect.

Consequently, there could be much more dust than has been suspected as the result of measurements using other techniques. Perley said that dark gamma-ray bursts could therefore provide a complementary way of answering the question of how much star formation is going on inside galaxies in the early Universe. The data indicate that the star formation rate in the early Universe was not as intense as previously thought. More research needs to be done to confirm this conclusion.

The team has submitted a paper about the study to The Astronomical Journal.

The W. M. Keck Observatory operates twin 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i. The two telescopes feature a suite of advanced instrumentation including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and a world-leading laser-guide-star adaptive optics system. The Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. For information please call 808.885.7887 or visit http://www.keckobservatory.org.

The Swift mission, equipped with a gamma-ray detector and X-ray, ultraviolet and optical telescopes, is operated by NASA’s Goddard Spaceflight Center.