Andrew Yee[_1_]
December 21st 07, 05:51 PM
Robert Naeye/Rob Gutro
Goddard Space Flight Center, Md. December 18, 2007
301-286-4453/301-286-4044
RELEASE 07-80
INTERGALACTIC "SHOT IN THE DARK" SHOCKS ASTRONOMERS
GREENBELT, Md. -- A team of astronomers has discovered a cosmic explosion
that seems to have come from the middle of nowhere -- thousands of
light-years from the nearest galaxy-sized collection of stars, gas, and
dust. This "shot in the dark" is surprising because the type of explosion, a
long-duration gamma-ray burst (GRB), is thought to be powered by the death
of a massive star.
"Here we have this very bright burst, yet it's surrounded by darkness on all
sides," says Brad Cenko of the California Institute of Technology, Pasadena,
Calif., lead author of the team's paper, which has been accepted for
publication in The Astrophysical Journal. "The nearest galaxy is more than
88,000 light-years away, and there's almost no gas lying between the burst
and Earth."
The blast was detected on January 25, 2007, by several spacecraft of the
Inter-Planetary Network. Observations by NASA's Swift satellite pinpointed
the explosion, named GRB 070125 for its detection date, to a region of sky
in the constellation Gemini. It was one of the brightest bursts of the year,
and the Caltech/Penn State team moved quickly to observe the burst's
location with ground-based telescopes.
Using the team's robotic 60-inch telescope at Palomar Observatory in Calif.,
the astronomers discovered that the burst had a bright and fast-fading
afterglow in visible light. This prompted them to observe the afterglow in
detail with two of the world's largest telescopes, the 8-meter Gemini North
telescope and 10-meter Keck I telescope, both near the summit of Hawaii's
Mauna Kea.
What came next was a total surprise. Contrary to experience with more than a
hundred previous GRBs, Gemini spectra revealed no signs of dense gas and
dust absorbing the light of the afterglow. A trace of magnesium revealed
that the burst took place more than 9.4 billion years ago, as deduced by the
shift in wavelength of the afterglow's light, and that the surrounding gas
and dust was more tenuous than the environment around any previous burst.
To further pin down the environment that could produce such an unusual
explosion, the group obtained Keck images of the location of GRB 070125 long
after its afterglow light had faded away. Surprisingly, the resulting images
showed no galaxy at this location. "A Keck image could have revealed a very
small, faint galaxy at that distance," says coauthor Derek Fox of Penn
State.
Astronomers have amassed a great deal of evidence that GRBs are triggered by
the explosive deaths of massive stars, which live very short lives. Because
of their short lifespans, massive stars don't have time to wander far from
their birthplaces, usually dense clouds of gas and dust inside
respectable-size galaxies. So GRB 070125 raises the perplexing question of
how a massive star could be found so far away from any galaxy.
"Big stars live fast and die young, without much time to move around," says
Fox. "So if this massive star died far away from any galaxy, the key
question is, how did it manage to be born there?" The formation of massive
stars requires similarly massive aggregations of gas and dust, which are
usually found in bright galaxies.
One possibility is that the star formed in the outskirts of an interacting
galaxy, as seen in the famous Hubble Space Telescope picture of the
"Tadpole" galaxy, UGC 10214. "In the local universe, about one percent of
star formation happens in tidal tails, on the outskirts of two interacting
galaxies," says Cenko. "So it might even make sense to find one in 100
gamma-ray bursts in such an environment."
If this idea is correct, it should be possible to detect the tidal tail
hosting GRB 070125 by taking a long exposure with the Hubble Space
Telescope. "That's definitely our next stop," says Cenko.
"Many Swift discoveries have left astronomers scratching their heads in
befuddlement," adds Swift lead scientist Neil Gehrels of NASA Goddard Space
Flight Center in Greenbelt, Md. "But this discovery of a long GRB with no
host galaxy is one of the most perplexing of all."
For related images to this story, please visit on the Web:
http://www.nasa.gov/centers/goddard/news/topstory/2007/intergalatic_shot.html
Goddard Space Flight Center, Md. December 18, 2007
301-286-4453/301-286-4044
RELEASE 07-80
INTERGALACTIC "SHOT IN THE DARK" SHOCKS ASTRONOMERS
GREENBELT, Md. -- A team of astronomers has discovered a cosmic explosion
that seems to have come from the middle of nowhere -- thousands of
light-years from the nearest galaxy-sized collection of stars, gas, and
dust. This "shot in the dark" is surprising because the type of explosion, a
long-duration gamma-ray burst (GRB), is thought to be powered by the death
of a massive star.
"Here we have this very bright burst, yet it's surrounded by darkness on all
sides," says Brad Cenko of the California Institute of Technology, Pasadena,
Calif., lead author of the team's paper, which has been accepted for
publication in The Astrophysical Journal. "The nearest galaxy is more than
88,000 light-years away, and there's almost no gas lying between the burst
and Earth."
The blast was detected on January 25, 2007, by several spacecraft of the
Inter-Planetary Network. Observations by NASA's Swift satellite pinpointed
the explosion, named GRB 070125 for its detection date, to a region of sky
in the constellation Gemini. It was one of the brightest bursts of the year,
and the Caltech/Penn State team moved quickly to observe the burst's
location with ground-based telescopes.
Using the team's robotic 60-inch telescope at Palomar Observatory in Calif.,
the astronomers discovered that the burst had a bright and fast-fading
afterglow in visible light. This prompted them to observe the afterglow in
detail with two of the world's largest telescopes, the 8-meter Gemini North
telescope and 10-meter Keck I telescope, both near the summit of Hawaii's
Mauna Kea.
What came next was a total surprise. Contrary to experience with more than a
hundred previous GRBs, Gemini spectra revealed no signs of dense gas and
dust absorbing the light of the afterglow. A trace of magnesium revealed
that the burst took place more than 9.4 billion years ago, as deduced by the
shift in wavelength of the afterglow's light, and that the surrounding gas
and dust was more tenuous than the environment around any previous burst.
To further pin down the environment that could produce such an unusual
explosion, the group obtained Keck images of the location of GRB 070125 long
after its afterglow light had faded away. Surprisingly, the resulting images
showed no galaxy at this location. "A Keck image could have revealed a very
small, faint galaxy at that distance," says coauthor Derek Fox of Penn
State.
Astronomers have amassed a great deal of evidence that GRBs are triggered by
the explosive deaths of massive stars, which live very short lives. Because
of their short lifespans, massive stars don't have time to wander far from
their birthplaces, usually dense clouds of gas and dust inside
respectable-size galaxies. So GRB 070125 raises the perplexing question of
how a massive star could be found so far away from any galaxy.
"Big stars live fast and die young, without much time to move around," says
Fox. "So if this massive star died far away from any galaxy, the key
question is, how did it manage to be born there?" The formation of massive
stars requires similarly massive aggregations of gas and dust, which are
usually found in bright galaxies.
One possibility is that the star formed in the outskirts of an interacting
galaxy, as seen in the famous Hubble Space Telescope picture of the
"Tadpole" galaxy, UGC 10214. "In the local universe, about one percent of
star formation happens in tidal tails, on the outskirts of two interacting
galaxies," says Cenko. "So it might even make sense to find one in 100
gamma-ray bursts in such an environment."
If this idea is correct, it should be possible to detect the tidal tail
hosting GRB 070125 by taking a long exposure with the Hubble Space
Telescope. "That's definitely our next stop," says Cenko.
"Many Swift discoveries have left astronomers scratching their heads in
befuddlement," adds Swift lead scientist Neil Gehrels of NASA Goddard Space
Flight Center in Greenbelt, Md. "But this discovery of a long GRB with no
host galaxy is one of the most perplexing of all."
For related images to this story, please visit on the Web:
http://www.nasa.gov/centers/goddard/news/topstory/2007/intergalatic_shot.html