Clouds Dominate the Galactic Halo
Using the exquisite sensitivity of the National Science Foundation’s
Robert C. Byrd Green Bank Telescope (GBT), astronomer Jay Lockman of the
National Radio Astronomy Observatory (NRAO) in Green Bank, W. Va., has
produced the best cross-section ever of the Milky Way Galaxy’s diffuse
halo of hydrogen gas. This image confirms the presence of discrete
hydrogen clouds in the halo, and could help astronomers understand the
origin and evolution of the rarefied atmosphere that surrounds our Galaxy.
“The first observations with the Green Bank Telescope suggested that the
hydrogen in the lower halo, the transition zone between the Milky Way
and intergalactic space, is very clumpy,” said Lockman. “The latest
data confirm these results and show that instead of trailing away
smoothly from the Galactic plane, a significant fraction of the hydrogen
gas in the halo is concentrated in discrete clouds. There are even some
filaments.”
Beyond the star-filled disk of the Milky Way, there exists an extensive
yet diffuse halo of hydrogen gas. For years, astronomers have
speculated about the origin and structure of this gas. “Even the
existence of neutral hydrogen in the halo has been somewhat of a
puzzle,” Lockman remarked. “Unlike the Earth’s atmosphere, which is hot
enough to hold itself up against the force of gravity, the hydrogen in
the halo is too cool to support itself against the gravitational pull of
the Milky Way.”
Lockman points out that some additional factor has to be involved to get
neutral hydrogen to such large distances from the Galactic plane. “This
force could be cosmic rays, a supersonic wind, the blast waves from
supernovae, or something we have not thought of yet,” he said.
Earlier this year, data taken with the newly commissioned GBT
demonstrated that rather than a diffuse mist or other ill-defined
feature – as many astronomers had speculated – the halo was in fact made
up of well-defined clouds.
“The discovery of these clouds, each containing 50-to-100 solar masses
of hydrogen and averaging about 100 light-years in diameter, challenged
many of the prevailing theories about the structure and dynamics of the
halo,” said Lockman.
The clouds were discovered about 25,000 light-years from Earth toward
the center of our Galaxy. The latest findings show the clouds extend at
least 5,000 light-years above and below the Galactic plane.
Though the initial studies by Lockman revealed the presence of these
clouds, the data were insufficient to conclusively show that they were
present throughout the entire halo. These latest results provide
valuable evidence that the earlier results were truly representative of
the entire halo. “The richness and variety of this phenomenon continues
to astound me,” remarked Lockman.
Lockman’s new studies also confirm that these clouds travel along with
the rest of the Galaxy, rotating about its center. These studies
clearly rule out the possibility that so-called “high-velocity clouds”
were responsible for what was detected initially. High-velocity clouds
are vagabond clumps of intergalactic gas, possibly left over from the
formation of the Milky Way and other nearby galaxies. “One thing that
is for certain is that these are not high-velocity clouds, this is an
entirely separate phenomenon,” said Lockman.
According to the researcher, the ubiquitous nature and dynamics of these
newly discovered clouds support the theory that they are condensing out
of the hot gas that is lifted into the halo through supernova
explosions. When a massive star dies, it 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 will rise into the
Milky Way’s halo.
The results presented by Lockman suggest that, as some astronomers have
predicted, the hot gas in the halo slowly cools and condenses into
hydrogen clouds along with wispy filaments that connect them. When
these clouds become as massive as many of those discovered by Lockman,
they should then begin to fall back onto the Galactic plane. This
phenomenon is commonly referred to as a “galactic fountain.”
“If the clouds were part of the galactic fountain process,” Lockman
said, “then it is likely that they are now falling back onto the Galaxy.”
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 a wavelength of 21 centimeters.
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
National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated
under cooperative agreement by
Associated Universities, Inc.
