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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.
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.
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.
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.)
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.
Recent observations of atomic hydrogen gas suggest that
some of these filamentary structures are part of massive cloud falling
in toward the Galactic Center.
QuickTime Movie (2.1 MB);
Sound File (1.1 MB);
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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?
JPEG Image (11K); Caption, Credit and Copyright
JPEG Image (22K); Caption, Credit and Copyright
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
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.
QuickTime Movie (3.1 MB);
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