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Gamma-Ray Burst Illuminates Galaxy in Dark Ages

August 6, 2013

Before light from the gamma-ray burst arrives at the Earth for astronomers to study, it passes through interstellar gas in its host galaxy (close-up view, left), and intergalactic gas between the distant galaxy and us (wide view, right). This gas absorbs some colors and leaves a signature on the light that can be seen in its spectrum. This “signature” allows scientists to characterize the GRB, its environment, and the material between us and the distant galaxy. Credit: Gemini Observatory/AURA, artwork by Lynette Cook

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Gemini observatory’s rapid-response allows for the detailed study of light from one of the most distant cases of a gamma-ray burst illuminating a galaxy ever observed.

Thanks to Gemini’s unparalleled rapid-response capability, and ultra-sensitive nod-and-shuffle spectroscopy, radiation from a distant gamma-ray burst explosion clearly revealed a remarkable event near the edge of the observable universe. The Gemini optical (visible-light) spectrum provided critical evidence that the explosion happened in one of the most distant galaxies ever probed using the illumination provided by a gamma-ray burst (identified as GRB 130606A). This characterization is possible because the high-energy beacon of light from the GRB explosion must pass through interstellar gas within its young host galaxy (shown at left in the figure). The figure also illustrates how the light from the GRB is filtered by billions of light years of intergalactic gas that spans the space between us and the GRB (wide view, at right).

Such a distant GRB is particularly valuable as a window into the early universe. These observations are sensitive enough to be able to detect a fundamental change in the conditions, and instead show that the ionized state persisted until the universe was about one billion years old. Moreover, GRBs may prove to be a fair probe of these early conditions, unlike quasars, which may suffer from some biases.

The Gemini observations were obtained within about 13 hours of the GRB's discovery in early June of this year. The Gemini Multi-Object Spectrograph (GMOS) at Gemini North on Hawaii’s Mauna Kea captured the spectrum used in this research utilizing the instrument's extremely sensitive nod and shuffle mode. "Gemini gave us an exquisite intergalactic core-sample using the light from the GRB," said team-leader Ryan Chornock of Harvard University. "Gemini's nod and shuffle capability provided the best signal to noise ever on a high-redshift GRB spectrum and clearly shows the effect of the intergalactic medium on its light since intergalactic gas strongly absorbs the light from the GRB at most optical wavelengths.” Chornock adds that the nod and shuffle mode allowed researchers to carefully subtract emission from the night sky and make a precise measurement of the small fraction of light from the GRB transmitted through the intergalactic gas. The research also includes data from other observatories as described in the text below, excerpted from a press release issued by the Harvard-Smithsonian Center for Astrophysics on August 6, 2013.

Explosion Illuminates Invisible Galaxy in the Dark Ages

More than 12 billion years ago a star exploded, ripping itself apart and blasting its remains outward in twin jets at nearly the speed of light. At its death it glowed so brightly that it outshone its entire galaxy by a million times. This brilliant flash traveled across space for 12.7 billion years to a planet that hadn’t even existed at the time of the explosion – our Earth. By analyzing this light, astronomers learned about a galaxy that was otherwise too small, faint and far away for even the Hubble Space Telescope to see.

“This star lived at a very interesting time, the so-called dark ages just a billion years after the Big Bang,” says lead author Ryan Chornock of the Harvard-Smithsonian Center for Astrophysics (CfA).

“In a sense, we’re forensic scientists investigating the death of a star and the life of a galaxy in the earliest phases of cosmic time,” he adds.

The star announced its death with a flash of gamma rays, an event known as a gamma-ray burst (GRB). GRB 130606A was classified as a long GRB since the burst lasted for more than four minutes. It was detected by NASA’s Swift spacecraft on June 6th. Chornock and his team quickly organized follow-up observations by the MMT Telescope in Arizona and the Gemini North telescope in Hawaii.

“We were able to get right on target in a matter of hours,” Chornock says. “That speed was crucial in detecting and studying the afterglow.”

A GRB afterglow occurs when jets from the burst slam into surrounding gas, sweeping that material up like a snowplow, heating it, and causing it to glow. As the afterglow’s light travels through the dead star’s host galaxy, it passes through clouds of interstellar gas. Chemical elements within those clouds absorb light at certain wavelengths, leaving “fingerprints.” By splitting the light into a rainbow spectrum, astronomers can study those fingerprints and learn what gases the distant galaxy contained.

All chemical elements heavier than hydrogen, helium, and lithium had to be created by stars. As a result those heavy elements, which astronomers collectively call “metals,” took time to accumulate. Life could not have existed in the early universe because the elements of life, including carbon and oxygen, did not exist.

Chornock and his colleagues found that the GRB galaxy contained only about one-tenth of the metals in our solar system. Theory suggests that although rocky planets might have been able to form, life probably could not thrive yet.

“At the time this star died, the universe was still getting ready for life. It didn’t have life yet, but was building the required elements,” says Chornock.

At a redshift of 5.9, or a distance of 12.7 billion light-years, GRB 130606A is one of the most distant gamma-ray bursts ever found.

The team’s results will be published in the Sept. 1 issue of The Astrophysical Journal and are available online.