Press Release

GBT Reveals Satellite of Milky Way in Retrograde Orbit

By SpaceRef Editor
May 22, 2003
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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 a cooperative agreement with
Associated Universities, Inc.


Editors: An artist’s conception of Complex H is available from the
on-line press release.

Additional Contact Information:
Jay Lockman: 304-456-2302

SpaceRef staff editor.