Wide Field Radio Imaging of the Galactic Center

11/23/1998 - 56-98r
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Dr. N.E. Kassim and collaborators at the Naval Research Laboratory (NRL) in Washington, D.C., have produced the largest and most sensitive radio image ever made of the Milky Way's center at a uniform and high resolution. Radio images are used to view the center of our Galaxy because it is hidden behind a thick veil of dust and gas and cannot be seen in visible light.

This new image has led to the discovery of many new features, including a new supernova remnant, numerous pulsar candidates, and several new filamentary or thread-like structures (actually called "threads"). The image also serves as a useful tool for astronomers because it displays all of the major components of the Milky Way's central region in a single image.

Follow-up observations of the filamentary structures have revealed one of these to be a previously undiscovered "thread." It is an important discovery because this new thread, newly named the Pelican is the farthest yet known from the Galactic center and is oriented nearly perpendicular to the previously known threads. The orientation is important because it is believed to trace the large-scale geometry of the magnetic field in the Galactic Center environment. Another new feature displaying a mixture of thread-like and supernova remnant-like morphology is newly named the Cane.

The NRL scientists, using data originally obtained by the National Radio Astronomy Observatory's Very Large Array radio telescope, produced this panoramic view of the center of our Milky Way Galaxy at a radio wavelength (color) of approximately one meter. The data were reprocessed on DoD high performance computing resources using specialized software that corrects for the distortions normally present in conventionally generated high-resolution, wide field-of-view images made at long wavelengths. This image marks the start of a series of ongoing observations intended to monitor the variable and transient sources at the Galactic center.

Basic research in radio and infrared astonomy at NRL is supported by the Office of Naval Research. An online atlas is available at http://rsd-www.nrl.navy.mil/7213/lazio/GC/.

Naval Research Laboratory
Photo caption Wide-Field Radio Image of the Galactic Center
Click on visual for a full-size version.

At approximately 2 degrees on a side (the full moon is about 1/2 degree across), or about 1000 light years at the distance of the center of the Milky Way, the image is shown in false color with brightness indicating areas of intense radio emission, and surrounding dark areas indicating less intense radio emission. (Dark lines near the center of the image are artifacts of the image processing.) The concentration of radio sources along a diagonal line through the image reveals the disk-like shape of the Milky Way viewed edge-on.

The most prominent source in the image is Sgr A. (Its name derives from the fact that the Milky Way's center is in the direction of the constellation Sagittarius, abbreviation Sgr.) Deep within Sgr A is the source Sgr A* (not visible in photo), which astronomers have identified as possibly being a black hole with a mass millions of times that of the Sun.

Sgr A and Sgr A* are clearly not the only sources, though. As hot young stars form, they heat the gas around them. Eventually, the gas can become hot enough that it glows, serving as a lamppost to show where stars are forming. There are a number of prominent regions of star formation in the Milky Way's center including Sgr B1 and B2 and part of Sgr D (Sgr D HII). When hot stars run out of fuel, they collapse, producing massive explosions known as supernovae. The explosive debris becomes a supernova remnant (SNR), within which are high speed electrons spiraling around magnetic fields. A number of such supernova remnants are visible within this image. In addition, this spiraling or synchrotron radiation seems to be responsible for a collection of enigmatic sources known as the Galactic center arc, threads, and the Snake. The true nature of these filamentary structures remains a mystery, though it is clear that their emission, orientation, and structure provide important clues to the energetics and large-scale magnetic field structure in the center of the Milky Way.

Not all sources visible are in the center of the Galaxy, though. Many of the bright spots, particularly those in Sgr E and near the edges of the image, are distant galaxies shining through our own Milky Way, says the scientific team. On the other hand, notes Dr. Kassim, the Mouse is thought to be a high-speed object between us and the center of the Milky Way.

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