New Radio Images of Milky Way's Center Revise View of Magnetic Fields
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(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.
About the U.S. Naval Research Laboratory
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