What are those mysterious gasses in our Milky Way? (Forwarded)

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Jan. 06, 2004

What are those mysterious gasses in our Milky Way?

ANN ARBOR, Mich. -- University of Michigan astronomers have found evidence for
clouds of high-speed gases around galaxies that may help scientists understand
the mysteries of similar material in our own Milky Way galaxy. Postdoctoral
fellow Eric Miller and astronomy professor Joel Bregman presented their results
Jan. 8 at the 203rd meeting of the American Astronomical Society in Atlanta,
Georgia, where they described their studies of the faint radio and optical
signals from two nearby galaxies.

The Milky Way galaxy has a disk of stars and gas that rotates in an orderly
fashion, but the galaxy also contains clouds of neutral gas that are not part of
this motion. "These high velocity clouds have been a mystery for some time,
mainly because we do not know their distance, which is necessary to determine
their mass," Miller said. Both mass and distance provide clues to the clouds'
origins. Observing clouds of this type in another galaxy solves the distance
problem because the clouds are at nearly the same distance as the galaxy being
observed. However, radio emissions from the clouds, which provide other
essential information for calculating cloud mass, are dim, presenting a challenge.

Miller and Bregman met the challenge by carefully selecting two galaxies to
study: Messier 51 (the Whirlpool galaxy) in the constellation Canes Venatici
(the Hunting Dogs) and Messier 83 in the constellation Hydra (the Water Snake)
-- two of the nearest spiral galaxies that resemble our own Milky Way galaxy. In
addition to being "magnificent" to view in optical light, "they are two of the
very best galaxies to study in radio waves, where we have a great view of their
disk of spinning neutral gas," Miller said. Since many of the high velocity
clouds appear projected on the disk, studying them is easier when the disk is
readily observed.

The high velocity clouds of neutral hydrogen that the astronomers were seeking
account for only about one percent of the gas in a galaxy. The rest of the gas
in the galaxy is also neutral, but rotates quietly around the galaxy. Like the
gas in the high velocity clouds, it gives off radio waves (similar to those used
in cordless telephones) that can be picked up with large radio telescopes.
Miller and Bregman used the Very Large Array of radio telescopes at the National
Radio Astronomy Observatory's site in New Mexico. These 27 radio dishes,
arranged in the shape of a Y that is 1 km (0.62 mile) across, work together to
produce images of the sky in radio light.

"One of the big challenges was to identify faint blobs of emission and make sure
that we weren't confusing it with radio noise," Miller said. But by recording
images of the gases at different velocities, the astronomers were able to
distinguish clouds that were moving peculiarly from the rest of the calmly
moving gas. "We were able to identify individual high velocity clouds in these
galaxies for the first time, and we also found that many more smaller clouds are
present," said Miller. In the galaxy Messier 83, the clouds appear to be
projected on the galaxy disk, which indicates that the galaxy is making the
clouds, said Bregman. "These clouds are probably produced by a galactic
fountain, which occurs when normal disk gas is heated by supernovae (exploding
stars), rises out of the disk, cools and falls back as clouds of neutral gas.
It's like a galactic weather pattern where the clouds are like snow or hail."

The astronomers' observations of the Whirlpool galaxy help explain the origin of
high velocity clouds in the Milky Way. "If the Whirlpool galaxy has the same
amount of high velocity gas as the Milky Way -- and there is evidence that this
is the case -- then we can use that information to solve for the distance of the
high velocity clouds in the Milky Way galaxy," Miller said. "When we do that,
the distance we obtain places the clouds in the halo of the Milky Way,"
suggesting that the clouds formed from the galaxy, possibly when another galaxy
passed nearby, pulling off outer parts from each galaxy and creating long
streamers of gas.

A competing idea is that the clouds are gas that never formed into stars and
that these clouds lay far from galaxies. "Our observations argue against that
picture," Miller said.

Related links:

* Joel Bregman
http://www.astro.lsa.umich.edu/users/jbregman/
* National Radio Astronomy Observatory's Very Large Array
http://www.vla.nrao.edu/
* American Astronomical Society
http://www.aas.org/
* Additional images
http://www.astro.lsa.umich.edu/~milleric/hvcs/

IMAGE CAPTION:
[http://www.umich.edu/news/index.html...Dec03/clouds2]
The spiral galaxy M 83 as seen at radio wavelengths. The false color image shows
the neutral hydrogen gas, which is much more extended than the starlight in this
galaxy, shown at the same scale in the inset image. The black contours show the
locations of the high-velocity neutral gas clouds discovered by the team from
the University of Michigan. The largest clouds are found over the spiral arms of
the galaxy, where star formation is thought to be ejecting material out of the
plane of the galaxy.

Photo credit: University of Michigan and National Radio Astronomy Observatory