The Galactic Center

A Wide-Field, Low-Frequency Image

N. E. Kassim, D. S. Briggs (NRL), J. Imamura (TJHSST), T. J. W. Lazio (NRL), T. N. LaRosa (Kennesaw State Univ.), & S. D. Hyman (Sweet Briar College)

Introduction

Forever hidden behind a thick veil of dust and gas, the center of our Milky Way Galaxy cannot be seen in visible light, the kind of light that our eyes see. In order to study the center of our Galaxy, astronomers must turn to other colors of light, like gamma-rays, X-rays, infrared, and radio.

This panoramic view of the center of our Milky Way Galaxy is at a radio wavelength (color) of approximately 1 meter. This image was produced by N. Kassim and collaborators at the Naval Research Laboratory. This image is the largest and most sensitive radio image ever made of the Milky Way's center at a uniform and high resolution. (Larger annotated [757 kB JPEG] and non-annotated [2.3 MB] versions of this image are also available for download.)

The beauty and complexity of this image excites the imagination of experts and non-experts alike. The concentration of sources along a diagonal line through the image reveals the disk-like shape of the Milky Way viewed edge-on. This image also serves as a useful tool for astronomers because it displays all of the major components of the Milky Way's center region in a single image (see also the schematic).

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*, which astronomers have identified as possibly being a black hole with a mass millions of times that of the Sun.

Sgr A is clearly not the only source, 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. When hot stars run out of fuel, they collapse, producing massive explosions known as supernovae. The explosive debris becomes a supernova remnant, 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 enigmimatic sources known as the Galactic center arc, filaments, and threads. 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.

This image has also led to the discovery of many new sources, including a new supernova remnant, numerous pulsar candidates, and several new filamentary structures. We are also using this image as the initial epoch in a series of ongoing observations intended to monitor the variable and transient source populations at the Galactic center.

Image

This image shows the central 4° x 4° of the Galaxy at a radio frequency of 330 MHz (wavelength 0.92 meters). (For comparison the full Moon is 1/2 degree in diameter.) It was produced from data acquired from the VLA's B-, C-, and D configurations, originally obtained by Pedlar, Anantharamiah, Goss, & Ekers. We re-processed the data through the NRAO Software Development Environment program dragon, which corrects for the distortions normally present in high resolution, wide-field images made at long wavelengths.

The resolution is 48" x 48". The peak brightness is 8.5 Jy/beam, and the rms noise level (excluding the bright sources on the Galactic ridge) is 5.9 mJy/beam. For additional details, please consult "A Wide-Field 90 Centimeter VLA Image of the Galactic Center Region" by LaRosa et al. (2000). You may also access a preprint copy of this paper and/or a FITS copy of Figure 1 from that paper.

The yellow boxes on the image indicate sources we have identified as being above 5 times the rms noise level. Clicking on one of these yellow boxes will provide further information about the source(s) in that region.

Schematic

This schematic identifies the major sources in the Galactic center. The Galactic equator (b = 0°) is shown horizontal, so the schematic is rotated as compared to the image above.

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Jennifer Imamura
T. Joseph W. Lazio
Namir E. Kassim
Last modified: Fri Feb 25 15:51:40 2005