Green Bank, WV – New studies with the National Science Foundation’s
Robert C. Byrd Green Bank Telescope (GBT) have revealed a previously
unknown population of discrete hydrogen clouds in the gaseous halo that
surrounds the Milky Way Galaxy. These clouds were discovered in the
transition zone between the Milky Way and intergalactic space, and
provide tantalizing evidence that supernova-powered “galactic
fountains” continually blast superheated hydrogen gas into our galactic
suburbs.

Caption: GBT data have uncovered an entirely new component of the Milky
Way Galaxy — distinct clouds of neutral atomic hydrogen (HI) floating
above the plane of the Galaxy. Actual GBT data are shown in the cut
away above an edge-on artist’s rendition of our Galaxy. Credit: NSF/K.
Woellert; NRAO/AUI [Larger version]

Extending far above the star-filled disk of the Milky Way is an
atmosphere, or halo, of hydrogen gas. “By studying this halo, we can
learn a great deal about the processes that are going on inside our
Galaxy as well as beyond its borders,” said Jay Lockman, an astronomer
with the National Radio Astronomy Observatory (NRAO) in Green Bank, West
Virginia. “It has remained a mystery, however, how this halo formed and
what has prevented gravitational forces from collapsing the gas into a
thin layer long ago.”

Some astronomers have speculated that this gas is distributed as a
diffuse mist held up by either magnetic fields or cosmic rays streaming
out of the plane of the Milky Way. Others believed that it is made of
innumerable long-lived hydrogen clouds bobbing up and down like balls
tossed by a juggler.

Early observations with other telescopes discovered that there was some
neutral hydrogen gas floating far above the Galaxy’s plane, but these
instruments were not sensitive enough to reveal any structure or resolve
questions about its origin.

Lockman’s studies for the first time show a clear picture of the
structure of the gas. Rather than a mist, the halo is in fact full of
discrete clouds, each containing 50-to-100 solar masses of hydrogen and
averaging about 100 light-years in diameter. “These objects were just
below the ability of the older telescopes to detect,” said Lockman, “but
I looked with the GBT, and they popped right out.” Lockman’s results
will be published in the Astrophysical Journal Letters.

The clouds were discovered about 15,000 light-years from Earth toward
the center of our Galaxy, and about 5,000 light-years above the Galaxy’s
plane.

One of the most compelling facts revealed by the GBT is that the clouds
are coupled dynamically to the disk of the Galaxy; that is, they follow
along with the rotation of the rest of the Milky Way. Material from
other sources crashing into the Milky Way would have different
velocities and also appear quite different. “These are home grown
objects, and not interlopers from outside our own Galaxy,” said Lockman.

Although the origin of these newly discovered clouds is not yet known,
one mechanism to explain how this gas could be lifted into the halo is
through supernova explosions. When a massive star reaches the end of
its life it erupts in a cataclysm that produces a burst of cosmic rays
and an enormous expanding bubble of gas at a temperature of several
million degrees Celsius. Over time, this hot gas can flow outward into
the Milky Way’s halo.

The question remains, however, what happens to this gas once it’s
ejected into the halo. One possibility is that it leaves the Galaxy as
a wind, never to return. Some astronomers predict, however, that as the
gas slowly cools it would condense into hydrogen clouds, eventually
falling like raindrops back into the Milky Way, and forming what is
referred to as a galactic fountain.

“If the clouds were formed by material ejected from the Galactic plane
into the halo,” Lockman said, “then it’s possible that they are now
falling back onto the Galaxy. This would then require a continuing flow
of new material from supernova explosions into the halo to replenish the
hydrogen gas that has rained back into the disk.”

The researcher comments that further observations, now in progress,
should clarify the properties of these halo clouds, determine their
distribution throughout the Galaxy, show how they are related to other
types of clouds, and reveal their internal structure.

Radio telescopes are able to detect the naturally occurring radio
emission from neutral atomic hydrogen. As hydrogen atoms move about in
space, they can absorb small amounts of energy, sending the atom’s
single electron to a higher energy state. When the electron eventually
moves back to its lower energy — or resting state, it gives up a small
amount of electromagnetic radiation at radio frequencies. The
individual energy of a single atom is very weak, but the accumulated
signal from vast clouds of hydrogen is strong enough to be detected by
sensitive radio telescopes on Earth.

The GBT, dedicated in August of 2000, is the world’s largest fully
steerable radio telescope. Its 100 by 110 meter dish is composed of
2004 individually hinged panels. It also has a unique offset feed arm,
which greatly enhances the performance of the telescope, making it ideal
for observations of faint astronomical objects. The GBT is completing
its commissioning and early science program and will be moving into full
time operation.

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

Note to Editors:

Additional Contact: Jay Lockman 304-456-2302; jlockman@nrao.edu