Milky Way Galaxy is warped and vibrating like a drum (Forwarded)

Media Relations
University of California-Berkeley

Media Contacts:
Robert Sanders
(510) 643-6998, (510) 642-3734

Additional Resources:

Leo Blitz, UC Berkeley
(510) 643-3000

Martin Weinberg
University of Massachusetts, Amherst
(413) 545-3821

Evan Levine, UC Berkeley
(510) 643-8592

Carl Heiles, UC Berkeley
(510) 642-4510

FOR IMMEDIATE RELEASE: Monday, January 09, 2006

Milky Way Galaxy is warped and vibrating like a drum
By Robert Sanders, Media Relations

BERKELEY -- 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.

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.

They found that the atomic gas layer is vibrating like a drum, and that
the vibration consists almost entirely of three notes, or modes.

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.

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.

"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.

"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."

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. Blitz will
summarize the work later that day during a 12:30 p.m. press briefing in
the Wilson C Room of the Marriott Wardman Park Hotel.

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.

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.

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.

"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."

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.

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.

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.

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.

"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.

"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.

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.

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.

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.

"We often think of the warp as being static, but this simulation shows
that it is very dynamic," Blitz said.

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.

The research of the UC Berkeley group is supported by the National
Science Foundation. Weinberg is partly supported by NASA and the NSF.

IMAGE CAPTIONS:

[Figure 1:
(Top image)
http://astron.berkeley.edu/~blitz/Pr...figure1top.jpg (438KB)
(Bottom image)
http://astron.berkeley.edu/~blitz/Pr...ure1bottom.jpg (506KB)]
This is an image of the warped hydrogen layer of the Milky Way.

The colored contours are warped "up" with regard to the Galactic Plane,
the grey contours are warped "down." The top image is to scale; the
bottom image has the warp exaggerated by a factor of 5. The center of
the Milky Way is in the center of the white circular area, and the
position of the Sun is given by the dot with a circle around it. The Sun
is orbiting the center of the Galaxy and is moving to the upper right in
this view. The white circular area and the white wedges are areas that
are not amenable to analysis. Note how the warp is asymmetric. The
jagged features are of relatively small scale, localized features in the
warp. The disk of stars is largely, but not entirely contained within
the white circular area.

[Figure 2:
(Upper left image)
http://astron.berkeley.edu/~blitz/Pr...re2topleft.jpg (421KB)
(Upper right image)
http://astron.berkeley.edu/~blitz/Pr...e2topright.jpg (223KB)
(Lower left image)
http://astron.berkeley.edu/~blitz/Pr...bottomleft.jpg
(242KB)
(Lower right image)
http://astron.berkeley.edu/~blitz/Pr...ottomright.jpg
(408KB)]
This is an image of the 3 "notes" or modes that make up the vibration of
the hydrogen layer. The upper left is the fundamental tone, what
astronomers call m=0; it is shaped like a bowl. The upper right is the
first overtone, m=1; it is shaped like the brim of a hat. The lower left
is the second harmonic, m=2; it is shaped like a saddle. The three modes
together make up the warp that is shown in the lower left, which is a
good representation of the data without the jagged details. These
"notes" are about 3 million octaves below middle C.

[Figure 3:
http://astron.berkeley.edu/~blitz/Pr...se/figure3.png (62KB)]
This is a still from a movie of how the Magellanic Clouds produce the
warp. The Milky Way is in the middle. And the position of the Sun is
about half way out in the picture of the Galaxy along the line marked X.
The cross hatched area represents the warped hydrogen layer at the
present time. The looping line is the orbit of the Magellanic Clouds and
the position of the bead on the line represents the location of the
Clouds at the present time. The orbital period is about 1.5 billion years.

The movie may be found at the following URL:
http://astron.berkeley.edu/~blitz/Pr...ease/movie.avi (1.7MB)

Does anyone find it unlikely that the Magellanic Clouds have only
completed maximum of 8 orbits of The Milky Way? (1.5 by per orbit)

Jim G
c'=c+v

In message . com,
writes
Does anyone find it unlikely that the Magellanic Clouds have only
completed maximum of 8 orbits of The Milky Way? (1.5 by per orbit)

Jim G
c'=c+v


Given that the Milky Way is about 12 thousand million years old, and the
orbital period is 1.5 thousand million years, no.
Your point?

Jonathan Silverlight wrote:

In message . com,
writes
Does anyone find it unlikely that the Magellanic Clouds have only
completed maximum of 8 orbits of The Milky Way? (1.5 by per orbit)

Jim G
c'=c+v


Given that the Milky Way is about 12 thousand million years old, and the
orbital period is 1.5 thousand million years, no.
Your point?

There was no suggestion that the orbit (Clouds) is wobbly (read
unstable or highly eliptical),
which I thought was reasonable to expect of such a new system. Unless
Milky Way and Mag. more or less formed together?
So my point.......are the ages/timescales anywhere near correct?

Jim G
c'=c+v