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The center of our own Milky Way Galaxy is a complex and mysterious place, shrouded by dust from the view of optical telescopes. The dust makes radio observations critically important to understanding this region. The VLA was first used to study the Galactic Center in 1981, and, with its sharp, sensitive, radio "vision," continues to be one of astronomers' primary tools for observing the heart of our Galaxy.
In 1983, a team of observers led by Ron Ekers used the VLA to make an image of the Galactic Center that revealed for the first time a "mini-spiral" of hot gas. That same year, K.Y. Lo and Mark Claussen made an even more-detailed image of the region that showed not only the mini-spiral but also a distinct, separate point of radio emission corresponding to the exact center of the Galaxy. That point is known to astronomers as Sgr A*, and had been discovered in 1974 by observers using NRAO's Green Bank Interferometer in West Virginia. The 1983 VLA image was the first to show it clearly separated from other structures in the Galactic Center. Today, Sgr A* is believed to be the site of a black hole about 2.6 million times more massive than the Sun.
Don Backer and Dick Sramek observed Sgr A* with the VLA over 16 years, from 1982 to 1998, measuring its slow apparent motion across the sky. Their purpose was to set limits on the mass of Sgr A*. If Sgr A* is comparable in mass to a star, it would be seen to move rapidly around the center of the Galaxy. On the other hand, if it is a very massive object, such as a black hole, it will appear to be at rest in the Galaxy's center.
Backer and Sramek's long-term project showed that Sgr A* itself appears to move very little, and they concluded that the object has a minimum mass of two million times that of the Sun. Observations with NRAO's Very Long Baseline Array, capable of even better measurements, confirm the VLA data. All this supports the prevalent idea of a supermassive black hole at the Galaxy's center.
The observations of Sgr A* also yielded another result. Part of the apparent motion of this object is due not to any actual motion on its part, but to the movement of our own Solar System in its orbit around the center of the Galaxy. At a distance of 26,000 light-years from the Galactic Center, the Solar System completes a Galactic orbit in about 226 million years. Despite that long time period, the VLA is capable of detecting such motion.
In 1984, Farhad Yusef-Zadeh, Mark Morris and Donald Chance discovered multiple, parallel filaments at the Galactic Center. At first, the scientists thought that the filaments were not real, but rather the result of bad data, but when the filaments showed up in additional VLA images made at a different observing frequency, their reality was confirmed. Since then, the VLA has revealed additional filaments and "threads." The filaments and threads are interpreted as magnetic features, and thus provide valuable information on the structure of magnetic fields at the Galaxy's center.
Several Galactic Center observers have used the VLA to study radio spectral lines, the distinctive "fingerprints" of atoms and molecules. These studies have revealed important data on gas velocities, allowing scientists to learn more about the relationships among different structures. Other data about velocities in the Galactic Center region has come from VLA observations of radio emissions from stars that are not visible to optical telescopes because of the dust obscuration.
The center of the Milky Way is a fascinating region, still not well understood, and will continue to be a popular observing target for the VLA for many years to come.
To see the "big picture" of the Galactic Center region, look at A VLA "Road Map" image of the Galactic Center, made by astronomers at the Naval Research Laboratory. This image has labels pointing out the principal features.