New Radio Images of Milky Way’s Center Revise View of Magnetic Fields

American Astronomical Society meeting, Seattle, Washington —
Naval Research Laboratory (NRL) astronomers using the National
Science Foundation’s Very Large Array (VLA) radio telescope to
make a new panoramic image of the center of our Milky Way
Galaxy have discovered new features that challenge the
conventional wisdom about the magnetic field at the Galaxy’s
center.

A new image of the Galactic center made by collecting radio
waves with a wavelength of about one meter has revealed
filament-like structures with a wide variety of orientations.
Such filaments have been seen before in radio images, but
the earlier ones all point in nearly the same direction.
The new image, which triples the number of such filaments
known, suggests that the magnetic field at the Galaxy’s
center is tangled like a bowl of spaghetti, rather than
well-ordered like that of a bar magnet.

“The Milky Way’s center is an exciting, mysterious region
that, once again, has given us a surprise,” said Dr. Namir
Kassim, an NRL astronomer and one of the leaders of the
research team.

Kassim and Dr. Joseph Lazio, also of NRL, worked with Mr.
Michael Nord, a graduate student at the University of
New Mexico doing his dissertation research at NRL, and
Professors Scott Hyman of Sweet Briar College (VA),
Theodore LaRosa of Kennesaw State University (GA),
Nebojsa Duric of the University of New Mexico and Dr.
Crystal Brogan of the National Radio Astronomy Observatory.
The scientists presented their results to the American
Astronomical Society meeting in Seattle, WA.

The new view of the Milky Way’s center came from a project
that used the VLA to make images at 1- and 4-meter
wavelengths, the longest wavelengths at which the VLA can
observe.

Forever hidden behind a thick veil of dust and gas, the
Milky Way’s center cannot be seen in visible light. In
order to study this region, astronomers must turn to
other forms of light, such as gamma-rays, X-rays, infrared,
and radio. “One of the key advantages of observing at long
radio wavelengths is that we can see the big picture,”
said Kassim. Observations at other wavelengths such X-ray
and infrared typically focus on individual objects;
large-scale images help place individual objects in a
broader context.

For nearly two decades, so-called “nonthermal filaments”
have been observed in the Milky Way’s center. While it is
clear that they are produced by the interaction between a
magnetic field and ultra-high velocity electrons, little
else is known about these enigmatic objects. In particular,
why do these magnetic filaments appear only in the Milky
Way’s center and apparently nowhere else?

One of the few things that astronomers thought they
understood about the filaments was what they implied about
the magnetic field in the Milky Way’s center. All of the
known filaments had been observed to point in nearly the
same direction. This led many astronomers to think that
the filaments were like iron filings near a bar magnet,
lining up along the magnetic field. As a result, most
astronomers favored a picture in which the magnetic field
in the Milky Way’s center was fairly simple, similar to
the Earth’s, and looking largely like a big bar magnet.

“Looking at our new 1-meter image, I was struck by a
number of filament-like structures pointing in the ‘wrong’
directions,” said Nord. The variety of their orientations
challenges the conventional wisdom about the magnetic
field in the Milky Way’s center but strengthens a model
developed by Prof. LaRosa: The magnetic field in the
Milky Way’s center is instead a tangled mess. In total,
the NRL team has tripled the number of these enigmatic
magnetic filaments in the Milky Way’s center.

The tangled filaments were not the only surprise revealed
by the new images.

The sky is filled with brief bursts of light, if one only
has the “eyes” to see. About once a day, a brief flash of
gamma-ray light occurs. These “gamma-ray bursts” are now
known to be powerful explosions, probably from dying stars,
occurring in distant galaxies. Might the Milky Way harbor
weaker examples of exploding or flaring sources?
“Amazingly, even though the sky is known to be full of
objects ‘popping off’ at X- and gamma-ray wavelengths,
very little looking has been done to see if there are
‘radio bursts’,” says Lazio. “This despite the fact that
it is relatively easier for an astronomical object to
produce a ‘radio burst’ than a gamma-ray burst,” he added.

Almost serendipitously, Prof. Hyman found one such radio
burst in the team’s observations. Because they were not
looking for it initially, though, the radio burst had
faded by the time they realized what they had seen. Are
there more? The team is working actively to process new
observations dedicated to finding radio bursts. “To date,
we’ve not found any really obvious bursts, like our first
one,” says Lazio.

The new images also are helping the astronomers to build
a three-dimensional picture of our Galaxy’s central
region. Distances are notoriously difficult to measure
in astronomy, but Dr. Brogan explains that a comparison
of the team’s 1- and 4-meter wavelength images yields a
decidedly simple way of figuring out where things are:
“If an object “A” blocks the view of object “B,” then
object A must be in front of object B.”

The key to this technique is that an object’s appearance
can change dramatically between 1 and 4-meter wavelengths.
Some objects, in particular regions of ionized hydrogen
can change from being transparent or translucent at
1-meter wavelength to being opaque at 4-meter wavelength.

“This change is familiar to anybody who has used an AM/FM
radio,” says Lazio. At the radio wavelengths used by AM
radio stations, the Earth’s ionosphere, a region of
ionized gas between about 50 and 600 miles above the
surface, is opaque (even reflective). By contrast, FM
radio stations use shorter wavelength signals to which
the ionosphere is transparent or translucent. Thus,
long-distance reception is possible for AM radio stations
but not FM stations.

“An observer on the Moon could detect our FM radio
stations but not our AM radio stations. Such an observer
would conclude, correctly, that our radio stations must
be behind the Earth’s ionosphere. In a similar fashion,
if we see an astronomical object at 1-meter wavelength,
but a huge cloud of ionized gas blocks our view of it at
4-meter wavelength, we conclude that the object is behind
the cloud of gas,” says Lazio.

“In many cases, we know how far away the cloud of gas is.
Knowing that other objects are behind it, we are building
up a three-dimensional map of the Milky Way toward our
Galactic center,” continues Kassim.

NRL is leading an effort to build a low-frequency radio
telescope far more sensitive than the VLA, named the Low
Frequency Array (LOFAR). “When completed, we expect it to
enable us to see hundreds of filaments and transients and
make a quantum leap in our 3-D model. LOFAR will
revolutionize many of these studies,” explains Kassim.
For more information visit
http://www.lofar.org

The National Radio Astronomy Observatory is a facility of
the National Science Foundation operated under cooperative
agreement by Associated Universities, Inc. Basic research
in radio astronomy at the NRL is supported by the Office
of Naval Research.

Images of Galactic center:
http://www.pao.nrl.navy.mil/pressRelease.php?Y=2003&R=1-03r-images