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National Radio Astronomy Observatory
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Charlottesville, VA 22903
http://www.nrao.edu

Contact:

Charles Blue, Public Information Officer, Charlottesville, VA
(434) 296-0323, cblue{at}nrao.edu

For Release: Thursday, May 22, 2003

GBT Reveals Satellite of Milky Way in Retrograde Orbit

New observations with National Science Foundation's Robert C. Byrd
Green Bank Telescope (GBT) suggest that what was once believed to be
an intergalactic cloud of unknown distance and significance, is
actually a previously unrecognized satellite galaxy of the Milky Way
orbiting backward around the Galactic center.

Jay Lockman of the National Radio Astronomy Observatory (NRAO) in
Green Bank, West Virginia, discovered that this object, known as
"Complex H," is crashing through the outermost parts of the Milky Way
from an inclined, retrograde orbit. Lockman's findings will be
published in the July 1 issue of the Astrophysical Journal, Letters.

"Many astronomers assumed that Complex H was probably a distant
neighbor of the Milky Way with some unusual velocity that defied
explanation," said Lockman. "Since its motion appeared completely
unrelated to Galactic rotation, astronomers simply lumped it in with
other high velocity clouds that had strange and unpredictable
trajectories." 

High velocity clouds are essentially what their name implies, 
fast-moving clouds of predominately neutral atomic hydrogen. They are 
often found at great distances from the disk of the Milky Way, and
may be left over material from the formation of our Galaxy and other
galaxies in our Local Group. Over time, these objects can become 
incorporated into larger galaxies, just as small asteroids left over 
from the formation of the solar system sometimes collide with the
Earth.

Earlier studies of Complex H were hindered because the cloud
currently is passing almost exactly behind the outer disk of the
Galaxy. The intervening dust and gas that reside within the sweeping
spiral arms of the Milky Way block any visible light from this object
from reaching the Earth. Radio waves, however, which have a much
longer wavelength than visible light, are able to pass through the
intervening dust and gas. 

The extreme sensitivity of the recently commissioned GBT allowed
Lockman to clearly map the structure of Complex H, revealing a dense
core moving on an orbit at a 45-degree angle to the plane of the
Milky Way. Additionally, the scientist detected a more diffuse region
surrounding the central core. This comparatively rarefied region
looks like a tail that is trailing behind the central mass, and is
being decelerated by its interaction with the Milky Way.

"The GBT was able to show that this object had a diffuse 'tail'
trailing behind, with properties quite different from its main body,"
said Lockman. "The new data are consistent with a model in which this
object is a satellite of the Milky Way in an inclined, retrograde
orbit, whose outermost layers are currently being stripped away in
its encounter with the Galaxy."

These results place Complex H in a small club of Galactic satellites 
whose orbits do not follow the rotation of the rest of the Milky Way. 
Among the most prominent of these objects are the Magellanic Clouds, 
which also are being affected by their interaction with the Milky
Way, and are shedding their gas in a long stream.

Since large galaxies, like the Milky Way, form by devouring smaller 
galaxies, clusters of stars, and massive clouds of hydrogen, it is
not unusual for objects to be pulled into orbit around the Galaxy
from directions other than that of Galactic rotation.

"Astronomers have seen evidence that this accreting material can come
in from wild orbits," said Butler Burton, an astronomer with the NRAO
in Charlottesville, Virginia. "The Magellanic clouds are being torn
apart from their interaction with the Milky Way, and there are
globular clusters rotating the wrong way. There is evidence that
stuff was going every-which-way at the beginning of the Galaxy, and
Complex H is probably left over from that chaotic period."

The new observations place Complex H at approximately 108,000 
light-years from the Galactic center, and indicate that it is nearly 
33,000 light-years across, containing approximately 6 million solar 
masses of hydrogen.

Radio telescopes, like the GBT, are able to observe these cold, dark 
clouds of hydrogen because of the natural electromagnetic radiation 
emitted by neutral atomic hydrogen at radio wavelengths (21
centimeters).

Globular clusters, and certain other objects in the extended Galactic 
halo, can be studied with optical telescopes because the material in 
them has collapsed to form hot, bright stars.

The GBT is the world's largest fully steerable radio telescope. It
was commissioned in August of 2000, and continues to be outfitted
with the sensitive receivers and components that will allow it to
make observations at much higher frequencies.

The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.

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