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.
advertisement
