Nearby galaxies undergoing a furious pace of star formation also
emit lots of gamma rays, say astronomers using NASA's Fermi
Gamma-ray Space Telescope. Two so-called "starburst" galaxies, plus
a satellite of our own Milky Way galaxy, represent a new category
of gamma-ray-emitting objects detected both by Fermi and
ground-based observatories.
"Starburst galaxies have not been accessible in gamma rays
before," said Fermi team member Seth Digel, a physicist at SLAC
National Accelerator Laboratory in Menlo Park, Calif. "Most of the
galaxies Fermi sees are exotic and distant blazars, which produce
jets powered by matter falling into enormous black holes. But these
new galaxies are much closer to us and much more like our own."
Gamma rays are the most energetic form of light. Fermi has
detected more than a thousand point sources and hundreds of
gamma-ray bursts, but the satellite also detects a broad glow that
roughly follows the plane of our galaxy. This diffuse gamma-ray
emission results when fast-moving particles called cosmic rays
strike galactic gas or even starlight.
Cosmic rays are hyperfast electrons, positrons, and atomic
nuclei moving at nearly the speed of light. But, although Earth is
constantly bombarded by these particles, their origin remains a
mystery nearly a century after their discovery. Astronomers suspect
that the rapidly expanding shells of exploded stars somehow
accelerate cosmic ray particles to their fantastic energy.
"For the first time, we're seeing diffuse emission from
star-forming regions in galaxies other than our own," noted
Jürgen Knödlseder, a Fermi collaborator at the Center for
the Study of Space Radiation in Toulouse, France. He spoke to
reporters today at the 2009 Fermi Symposium, a Washington gathering
of hundreds of astrophysicists involved in the Fermi mission and
related studies. The meeting continues through Nov. 5.
Knödlseder revealed an image captured by Fermi's Large Area
Telescope (LAT) of a star-forming region known as 30 Doradus within
the Large Magellanic Cloud (LMC). Located 170,000 light-years away
in the southern constellation Dorado, the LMC is the largest of
several small satellite galaxies that orbit our own.
More stars form in the 30 Doradus "star factory" than in any
similar location in the Milky Way. "The region is an intense source
of gamma rays, and the diffuse emission we see with Fermi follows
the glowing gas we see in visible light," Knödlseder
explained.
The region lights up in gamma rays for the same reason the Milky
Way does -- because cosmic rays strike gas clouds and starlight.
But Fermi shows that the LMC's brightest diffuse emission remains
close to 30 Doradus and doesn't extend across the galaxy. This
implies that the stellar factory itself is the source of the cosmic
rays producing the glow.
"Star-forming regions produce lots of massive, short-lived
stars, which explode when they die," Digel said. "The connection
makes sense."
"The tangled magnetic fields near 30 Doradus probably confine
the cosmic rays to their acceleration sites," Knödlseder
said.
Fermi's LAT sees diffuse emission from the starburst galaxies
M82 and NGC 253, both of which were also seen this year by
ground-based observatories sensitive to gamma rays hundreds of
times more energetic than the LAT can detect. They do this by
imaging faint flashes in the upper atmosphere caused by the
absorption of gamma rays carrying trillions of times the energy of
visible light.
"The core of M82 forms stars at a rate ten times greater than
the entire Milky Way galaxy," said Niklas Karlsson, a postdoctoral
fellow at Adler Planetarium in Chicago. He is also a member of the
science team for VERITAS, an array of gamma-ray telescopes in
Arizona that detected M82, which lies 12 million light-years away
in the constellation Ursa Major.
"These very-high-energy gamma rays probe physical processes in
other galaxies that will help us understand how and where cosmic
rays become accelerated," Karlsson explained.
"Our sensitivity to gamma-rays -- both in space and on the
ground -- has increased enormously thanks to Fermi and
observatories like VERITAS," Digel said. "This is opening up the
detailed study of high-energy processes in galaxies very close to
home." NASA's Fermi Gamma Ray Space Telescope is an astrophysics
and particle physics partnership, developed in collaboration with
the U.S. Department of Energy, along with important contributions
from academic institutions and partners in France, Germany, Italy,
Japan, Sweden and the United States.
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