Huge Cosmic Explosions Are Dark and Mysterious

PASADENA, CALIF. ? Some of the most powerful explosions in the universe are invisible. But astronomers are a sneaky bunch. By monitoring X-rays and gamma rays, they're able to see what's going on.

Today astronomers said that a certain type of gamma-ray burst, the most energetic explosions in the universe, can light up areas of galaxies, but only in these more energetic wavelengths of the electromagnetic spectrum, revealing intense star formation and death.

A survey of so-called "dark" gamma-ray bursts, which shine brightly in the gamma and X-ray parts of the spectrum but show barely a spark of visible light, found that these beacons can shed light on the dusty corners of galaxies where stars are born.

"Our study provides compelling evidence that a large fraction of star formation in the universe is hidden by dust in galaxies that do not appear otherwise dusty," said team member Joshua Bell of the University of California, Berkeley.

The findings were announced here today at the 214th meeting of the American Astronomical Society.

Star formation occurs in dense clouds that quickly fill with dust as the most massive stars rapidly age and explode, spewing newly created heavier elements into the interstellar medium to seed the formation of second-generation stars.

But measuring how much dust has built up in this process in the most distant galaxies is difficult. Gamma-ray bursts could aid that accounting process.

Long-duration gamma-ray bursts are thought to originate from the explosion of massive stars, creating two pencil-like beams of light bright enough to be seen from as far away as 13 billion light-years, near the limits of the observable universe.

While most gamma-ray bursts have a bright afterglow in the visible spectrum, some seem to lack this feature entirely, leaving investigators puzzled for the last 10 years.

Some speculated that these "dark" gamma-ray bursts were simply so far away that their visible afterglow shifted out of the optical range through a well-known process called redshift: Light from objects moving away from us shifts to the red end of the spectrum as its wavelengths are stretched. The shift, known as the Doppler phenomenon, is experienced on Earth when sound waves from an ambulance change pitch when the ambulance moves toward you vs. away from you.

But the new study, which focuses on 14 "dark" gamma-ray bursts, found that each of these bursts had a host galaxy perfectly visible to optical telescopes. If redshift were causing the bursts to appear "dark," optical telescopes wouldn't be able to see the host galaxies either.

Instead, the study showed that "dark" gamma-ray bursts have afterglows like any other, but dusty patches in the galaxies obscure the signal.

"We think we've solved most of the mystery of what actually makes them dark," said study leader Daniel Perley, a UC Berkeley graduate student.

Earlier this year, the team did find one "dark" gamma-ray burst that seems to be dark because of the redshift effect. The burst lit up in April and triggered telescopes. So while these type of "dark" gamma-ray bursts do exist, they are rare, Perley said.

The findings suggest that gamma-ray bursts may be able to help track the rate at which stars form and die in distant galaxies, and confirm previous estimates that "25 percent of the time, when massive stars form, they form in a dusty place,? Perley said.

"The dust is probably in clouds and knots around the forming stars," he added.

The study indicates that there could be much more dust than has been suspected as a result of measurements using other techniques, and "dark gamma-ray bursts could provide a complementary way of answering the question of how much star formation was going on inside galaxies in the early universe," Perley said.

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