By Ron Cowen
WASHINGTON — Astronomers may have detected smoke signals generated by a group of supernovas that blew up when the universe was less than 1.2 billion years old. If correct, the researchers have detected what would be one of the earliest known signs of supernova-produced dust in the universe, and the earliest dust detected thanks to a gamma-ray burst.
The exploding stars are too faint and lie too far away to be seen directly. But the smoke, or stardust, they produced when they erupted now appears to have been detected. The brilliant afterglow of a much more powerful type of eruption, a gamma-ray burst, has revealed the dust’s existence, a new analysis shows.
The new finding “is exciting because gamma-ray bursts are showing themselves as a unique probe of the early universe that really hadn’t been considered,” said Joshua Bloom of the University of California, Berkeley, reporting the work January 4 at the winter meeting of the American Astronomical Society. “We can now measure how matter in the [young] universe converts from stars into stardust,” the raw material for the next generation of stars, he added.
Bloom, Daniel Perley of UC Berkeley and their colleagues also posted the findings online. Stardust is known to absorb visible and ultraviolet light and to re-emit the light at longer, infrared wavelengths.
Gamma-ray bursts are mammoth explosions featuring the outpouring of jets of gamma rays, and those lasting more than a second or two are believed to mark the catastrophic collapse of a massive star into a black hole.
When the UC Berkeley astronomers and their colleagues examined the visible and infrared afterglows of a gamma-ray burst discovered in 2007 and dubbed GRB 071025, they found telltale light absorptions, indicating the presence of dust. Bloom said that the team tried several models to account for the properties of the afterglow light from the gamma-ray burst, but only dust from supernovas fit the bill.
In nearby reaches of the universe, including the Milky Way, most dust is produced by mature, low-mass stars. But at much earlier times in the universe, such as the era when GRB 071025 exploded about a billion years after the Big Bang, such stars were not yet common, Bloom said. Instead, he notes, supernovas, which are known to produce some dust in the modern universe, were the dominant — and perhaps only — source of dust.
The team suggests that light from this particular gamma-ray burst may have happened to pass through an unusually dusty part of its home galaxy, where a series of supernovas had popped off and enriched the surrounding space with dust grains.
This time isn’t the first that researchers have used a brilliant beacon of light to explore the dust content of the early universe. In 2004, Roberto Maiolino of the INAF-Astronomical Observatory of Rome and his colleagues reported evidence that light from a quasar more distant than GRB 071025 and hailing from an era about 100 million years earlier had also passed through a region of its home galaxy that was chock-full of supernova-generated dust.
Tracing the dust content of the cosmos, especially the composition and abundance of dust in early times, can reveal volumes about the evolution of the universe and the formation of stars, Maiolino and his colleagues have noted.
Bruce Draine of Princeton University says the new findings provide convincing evidence of ancient dust. But Draine says he’s not convinced that most of this early dust is supernova-generated. His models suggest that supernovas produce relatively few grains, which then act as seeds for the growth of additional particles of dust from elements that would have been readily available in the interstellar medium at that time.
