- The most prominent of the Milky Way's satellite galaxies
- a pair of galaxies called the Magellanic Clouds - appears to be interacting
with the Milky Way's ghostly dark matter to create a mysterious warp in
the galactic disk that has puzzled astronomers for half a century.
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- The warp, seen most clearly in the thin disk of hydrogen
gas permeating the galaxy, extends across the entire 200,000-light year
diameter of the Milky Way, with the sun and earth sitting somewhere near
the crease. Leo Blitz, professor of astronomy at the University of California,
Berkeley, and his colleagues, Evan Levine and Carl Heiles, have charted
this warp and analyzed it in detail for the first time, based on a new
galactic map of hydrogen gas (HI) emissions.
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- They found that the atomic gas layer is vibrating like
a drum, and that the vibration consists almost entirely of three notes,
or modes.
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- Astronomers previously dismissed the Magellanic Clouds
- comprised of the Large and Small Magellanic Clouds - as a probable cause
of the galactic warp because the galaxies' combined masses are only 2 percent
that of the disk. This mass was thought too small to influence a massive
disk equivalent to about 200 billion suns during the clouds' 1.5 billion-year
orbit of the galaxy.
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- Nevertheless, theorist Martin D. Weinberg, a professor
of astronomy at the University of Massachusetts, Amherst, teamed up with
Blitz to create a computer model that takes into account the Milky Way's
dark matter, which, though invisible, is 20 times more massive than all
visible matter in the galaxy combined. The motion of the clouds through
the dark matter creates a wake that enhances their gravitational influence
on the disk. When this dark matter is included, the Magellanic Clouds,
in their orbit around the Milky Way, very closely reproduce the type of
warp observed in the galaxy.
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- "The model not only produces this warp in the Milky
Way, but during the rotation cycle of the Magellanic Clouds around the
galaxy, it looks like the Milky Way is flapping in the breeze," said
Blitz, director of UC Berkeley's Radio Astronomy Laboratory.
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- "People have been trying to look at what creates
this warp for a very long time," Weinberg said. "Our simulation
is still not a perfect fit, but it has a lot of the character of the actual
data."
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- The interaction of the Magellanic Clouds with the dark
matter in the galaxy to produce an enigmatic warp in the hydrogen gas layer
is reminiscent of the paradox that led to the discovery of dark matter
some 35 years ago. As astronomers built better and better telescopes able
to measure the velocities of stars and gas in the outer regions of our
galaxy, they discovered these stars moving far faster than would be expected
from the observed number and mass of stars in the entire Milky Way. Only
by invoking a then-heretical notion, that 80 percent of the galaxy's mass
was too dark to see, could astronomers reconcile the velocities with known
theories of physics.
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- Though no one knows the true identity of this dark matter
- the current consensus is that it is exotic matter rather than normal
stars too dim to see - astronomers are now taking it into account in their
simulations of cosmic dynamics, whether to explain the lensing effect galaxies
and galaxy clusters have on the light from background galaxies, or to describe
the evolution of galaxy clusters in the early universe.
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- Some physicists, however, have come up an alternative
theory of gravity called Modified Newtonian Dynamics, or MOND, that seeks
to explain these observations without resorting to belief in a large amount
of undetected mass in the universe, like an invisible elephant in the room.
Though MOND can explain some things, Weinberg thinks the theory will have
a hard time explaining the Milky Way's warp.
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- "Without a dark matter halo, the only thing the
gas disk can feel is direct gravity from the Magellanic Clouds themselves,
which was shown in the 1970s not to work," he said. "It looks
bad for MOND, in this case."
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- Because many galaxies have warped disks, similar dynamics
might explain them as well. Either way, the researchers say their work
suggests that warps provide a way to verify the existence of the dark matter.
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- The starting point for this research was new spectral
data released this past summer about hydrogen's 21-centimeter emissions
in the Milky Way. The survey, the Leiden-Argentina-Bonn or LAB Survey of
Galactic HI, merged a northern sky survey conducted by astronomers in the
Netherlands (the Leiden/Dwingeloo Survey) with a southern sky survey from
the Instituto Argentino de Radioastronomía. The data were corrected
by scientists at the Institute for Radioastronomy of the University of
Bonn, Germany.
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- Blitz, Levine and Heiles, UC Berkeley professor of astronomy,
took these data and produced a new, detailed map of the neutral atomic
hydrogen in the galaxy. This hydrogen, distributed in a plane with dimensions
like those of a compact disk, eventually condenses into molecular clouds
that become stellar nurseries.
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- With map in hand, they were able to mathematically describe
the warp as a combination of three different types of vibration: a flapping
of the disk's edge up and down, a sinusoidal vibration like that seen on
a drumhead, and a saddle-shaped oscillation. These three "notes"
are about 3 million octaves below middle C.
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- "We found something very surprising, that we could
describe the warp by three modes of vibration, or three notes, and only
three," Blitz said, noting that this rather simple mathematical description
of the warp had escaped the notice of astronomers since the warp's discovery
in 1957.
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- "We were actually trying to analyze a more complex
'scalloping' structure of the disk, and this simple, elegant vibrational
structure just popped out," Levine added.
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- The current warp in the gas disk is a combination of
these three vibrational modes, leaving one-half of the galactic disk sticking
up above the plane of stars and gas, while the other half dips below the
disk before rising upward again farther outward from the center of the
galaxy. The results of this analysis will be published in an upcoming issue
of the Astrophysical Journal.
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- Weinberg thought he could explain the observed warp dynamically,
and used computers to calculate the effect of the Magellanic Clouds orbiting
the Milky Way, plowing through the dark matter halo that extends far out
into the orbit of the clouds.
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- What he and Blitz found is that the clouds' wake through
the dark matter excites a vibration or resonance at the center of the dark
matter halo, which in turn makes the disk embedded in the halo oscillate
strongly in three distinct modes. The combined motion during a 1.5-billion-year
orbit of the Magellanic Clouds is reminiscent of the edges of a tablecloth
flapping in the wind, since the center of the disk is pinned down.
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- "We often think of the warp as being static, but
this simulation shows that it is very dynamic," Blitz said.
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- Blitz, Levine and Heiles are continuing their search
for anomalies in the structure of the Milky Way's disk. Weinberg hopes
to use the UC Berkeley group's data and analysis to determine the shape
of the dark matter halo of the Milky Way.
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- Levine, a graduate student, will present the results
of the work in Washington, D.C., on Jan. 9 during a 10 a.m. session on
galactic structure at the American Astronomical Society meeting.
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- http://www.physorg.com/news9704.html
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