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Giant Radio Jet Coming from Wrong Kind of Galaxy
Giant jets of subatomic particles moving at
nearly the speed of light have been found coming from thousands
of galaxies across the Universe, but always from elliptical
galaxies or galaxies in the process of merging -- until now.
Using the combined power of the Hubble Space Telescope, the Very
Large Array (VLA) and the 8-meter Gemini-South Telescope,
astronomers have discovered a huge jet coming from a spiral
galaxy similar to our own Milky Way.
"We've always thought spirals were the wrong kind of galaxy
to generate these huge jets, but now we're going to have to
re-think some of our ideas on what produces these jets,"
said William Keel, a University of Alabama astronomer who led the
research team. Keel worked with Michael Ledlow of Gemini
Observatory and Frazer Owen of the National Radio Astronomy
Observatory. The scientists reported their findings at the
American Astronomical Society's meeting in Seattle, Washington.
"Further study of this galaxy may provide unique insights on
just what needs to happen in a galaxy to produce these powerful
jets of particles," Keel said.
In addition, Owen said, "The loose-knit nature of the
cluster of galaxies in which this galaxy resides may play a part
in allowing this particular spiral to produce jets."
Astronomers believe such jets originate at the cores of galaxies,
where supermassive black holes
provide the tremendous gravitational energy to accelerate
particles to nearly the speed of light. Magnetic fields twisted
tightly by spinning disks of material being sucked into the black
hole are presumed to narrow the speeding particles into thin
jets, like a nozzle on a garden hose.
Both elliptical and spiral galaxies are believed to harbor
supermassive black holes at their cores.
The discovery that the jet was coming from a spiral galaxy dubbed
0313-192 required using a combination of radio, optical and
infrared observations to examine the galaxy and its surroundings.
The story began more than 20 years ago, when Owen began a survey
of 500 galaxy clusters using the National Science Foundation's
then-new VLA to make radio images of the clusters. In the 1990s,
Ledlow joined the project, making optical-telescope images of the
same clusters as part of his research for a Ph.D dissertation at
the University of New Mexico. An optical image from Kitt Peak
National Observatory gave a hint that this galaxy, clearly seen
with a jet in the VLA images, might be a spiral.
Nearly a billion light-years from Earth, 0313-192 proved an
elusive target, however. Subsequent observations with the VLA and
the 3.5-meter telescope at Apache Point Observatory supported the
idea that the galaxy might be a spiral but still were
inconclusive. In the Spring of 2002, astronauts installed the
Advanced Camera for Surveys on the Hubble Space Telescope. This
new facility produced a richly-detailed image of 0313-192,
showing that it is a dust-rich spiral seen almost exactly
edge-on.
"The finely-detailed Hubble
image resolved any doubt and proved that this galaxy is a
spiral," Ledlow said. Infrared images with the Gemini-South
telescope complemented the Hubble images and further confirmed
the galaxy's spiral nature.
Now, the astronomers seek to understand why this one spiral
galaxy, unlike all others seen so far, is producing the bright
jets seen with the VLA and other radio telescopes.
Several factors may have combined, the researchers feel.
"This galaxy's disk is twisted, and that may indicate that
it has been disturbed by a close passage of another galaxy or may
have swallowed up a companion dwarf galaxy," Keel said. He
added, "This galaxy shows signs of having a very massive
black hole at its core, and the jets are taking the shortest path
out of the galaxy's own gas."
Owen points out that 0313-192 resides in a cluster of galaxies
called Abell 428. The scientists have discovered that Abell 428
is not a dense cluster, but rather a loose collection of small
groups of galaxies.
In order to see the large jets so common to elliptical galaxies,
Owen said, "you may need pressure from a cluster's
intergalactic medium to keep the particles and magnetic fields
from dispersing so rapidly that the jet can't stay
together."
However, "A spiral won't survive in a dense cluster,"
Owen said. Thus, the looser collection of galaxy groups that
makes up Abell 428 may be "just the right environment to
allow the spiral to survive but still to provide the pressure
needed to keep the jets together."
In any case, the unique example provided by this jet-producing
spiral galaxy "raises questions about some of our basic
assumptions regarding jet production in galaxies," Owen
said.
The prominent spiral galaxy in the lower right is not related to
0313-192, but is more than 200 million light-years closer.