Washington,
DC--A slow-motion collision between mysterious dark matter and two of the Milky
Way's galactic neighbors may be causing our galaxy to warp like a vinyl record
left out in the hot Sun, scientists announced today.
Astronomers
have puzzled over the Milky Way's warped shape for nearly half a century but
have been unable to provide a convincing explanation for what might be causing
it.
The warp is
most clearly visible in a thin disk of hydrogen gas that extends across the
entire 200,000-light-year diameter of the Milky Way.
Viewed
sideways, one half of the hydrogen disk appears to stick up above our galaxy's
plane of stars and gas, while the other half dips below the plane for a bit and
then rises upward again farther away from the galaxy's center.
Old
theory, new twist
One early
explanation was that the gravity from two neighboring dwarf galaxies known as
the Magellanic
Clouds was causing the Milky Way to warp as the pair moved in their
1.5-billion-year orbit around our galaxy.
This
hypothesis was later dismissed after it was shown that the combined mass of the
two dwarf galaxies is only about 2 percent of the Milky Way's enormous hydrogen
disk--not nearly enough to cause the warp.
Now
researchers from the University of California, Berkeley have revived this old
theory but with a new twist.
Using
computer models, the team showed that the Magellanic Clouds could warp
the shape of the Milky Way, but only if they were moving through a thick halo
of hypothetical dark
matter.
Dark matter
can't be directly observed because it neither emits nor reflects visible light
or other electromagnetic radiation. However, its presence has been inferred
from the gravitational effect it has on visible matter such as stars and
galaxies. Astronomers believe that dark matter may make up as much as 90
percent of the mass in the universe.
The
computer model suggests that if the dwarf galaxies were moving through dark
matter, then their gravitational influence would be enhanced to the point where
they could cause warping similar to what has been observed.
And a
surprise
The model
also revealed another surprise.
"We often
think of the warp as being static, but this simulation shows that it is very
dynamic," said Leo Blitz, an astronomer from UC Berkeley who was involved in
the study.
The model
indicates that as the Magellanic Clouds interact with dark matter, they create
vibrations that cause the Milky Way's hydrogen disk to oscillate. The overall
effect is reminiscent of the edges of a tablecloth flapping in the wind, the
researchers said.
Since many
other galaxies are also warped, the researchers believe similar processes might
explain their shapes as well.
The study
was detailed here in a press conference at the 207th Meeting of the
American Astronomical Society and will be published in an upcoming issue of the
Astrophysical Journal.
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