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The Galactic Center

Sgr A: 20 centimeter image; VLA

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What lurks at the center of our galaxy? From the vantage point of some 30 thousand light years away it is difficult to determine. Intervening dust obscures the core, making it invisible to optical telescopes. Radiation in far-infrared and radio regions of the spectrum are less affected by dust and thus are better able to reveal some of the striking phenomena at the Galactic Center.

Among the unusual objects believed to lie at the exact center of our Galaxy is a compact, yet extremely bright source known as Sagittarius A*. Some astronomers argue that this point-like source reveals a black hole with a mass equivalent to several million Suns.

A Black Hole in the Galactic Center?

A black hole with a mass of millions of Suns? Sounds extraordinary enough but theorieticians have suggested that the formation of a massive black hole may happen quite normally during a galaxy's evolution. However, not all astronomers are convinced that a massive black hole actually exists at the center of the Milky Way. What would it take to persuade them to the contrary?

Indirect evidence supporting the presence of a massive, central black hole is provided by the Sagitarrius A* radio source (often abbreviated to Sgr A*). Its radiation is consistent with recent theories concerning how black holes form and might interact with surrounding matter.

Leo Blitz, University of Maryland, on-camera
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QuickTime Movie (2.1 MB); Sound File (1.1 MB); Text

Current theory predicts that material falling in toward a black hole will form into a disk, often referred to as an accretion disk. As the gravitational energy is converted to heat, the gas in the accretion disk should become tremendously hot, emitting X-rays. In addition, electrons inside the disk would be accelerated to close to the speed of light; when these high-speed electrons interact with powerful magnetic fields associated with the black hole, they emit radiowaves across the entire radio spectrum.

This theory suggests that the Sgr A* radio source could be associated with the accretion disk surrounding a black hole. Because the disk would be so compact (and because the Galactic Center is so far away), most radio telescopes detect it as point source.

The most direct way to determine if an object like Sgr A* is due to an actual massive black hole is to observe the gravitational effects on the motion of the surrounding gas and stars. Many attempts to measure the velocities of the surrounding material have supported the existence of a single massive object; however, that is not the only possible explanation for the observed data. Infrared observations have located a dense cluster of stars within a few light-years of the Galactic center. Since this cluster appears to contain millions of stars, its presence, not a black hole, may account for the gravitational effects seen in the surrounding material.

Is is possible to distinguish between these two alternatives and finally uncover what lurks at the Galactic Center? To answer this question, astronomers must measure the velocities of stars within distances from the suspected black hole that are at the limits of resolution of modern-day telescopes. To prove that a black hole lies at the galaxy's center, they must show that a mass several millions times greater than our Sun is contained in volume too small to be star cluster. Such observations are currently underway.

Gathering Clues From Many Wavebands

One important question that concerns astronomers is what powers the Sgr A* radio source. If it were due to a black hole surrounded by an accretion disk, radio emissions would have been extinguished long ago, unless the disk were "refueled" with new infalling material. But then where would the material come from, and how would it be transported to the center?

Observations in the centimeter and millimeter wavebands have yielded a variety of clues as to how refueling might be taking place. For example, several interacting objects have been detected in the region of Sgr A*. One of the objects is a giant molecular cloud or GMC.

Sgr A East: 20 centimeter continuum image

Also in the vicinity is Sgr A East. Shown in the 20 centimeter VLA image below, this oval, shell-like feature is thought to be a supernova remnant, material from which may help fuel an accretion disk. (The red point in the image is the Sgr A* radio source.)
JPEG Image (11K); Caption, Credit and Copyright

Sgr A West: 6 centimeter image; VLA

Closer to Sgr A* lies a collection of filamentary structures known as Sgr A West. The emission from Sgr A West, shown in greater detail in the 6 centimeter VLA image below, is due to ionized gas that was heated up by the numerous young, hot stars in the region.
JPEG Image (22K); Caption, Credit and Copyright

Recent observations of atomic hydrogen gas suggest that some of these filamentary structures are part of massive cloud falling in toward the Galactic Center.

The Circumnuclear Ring

SgrA West: composite image (millimeter and centimeter)

Using the BIMA array, astronomers are studying the motions of gas surrounding Sgr A*. To map the locations of gas clouds and their corresponding velocities, they have taken advantage of the array's high spatial and spectroscopic resolutions. In doing so, scientists have discovered a ring of molecular gas centered upon Sgr A*.
JPEG Image (52K); Caption, Credit and Copyright

Leo Blitz, on-camera
Movie/Sound Byte
QuickTime Movie (3.1 MB); Sound File (1.7 MB); Text

Researchers have suggested that collisions between clouds within the ring could cause some of the gas to fall inward toward the center, thus feeding fuel to Sgr A*. Further observations in the millimeter and other wavebands will reveal more details about the ring and its interactions with the surrounding molecular clouds, as well as the mysterious, compact object lurking at the galactic center.

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Copyright © 1995: Board of Trustees, University of Illinois

NCSA. Last modified 11/11/95