In the outer parts of spiral galaxies, most of the mass in the
interstellar medium consists of neutral atomic hydrogen gas, with
traces of other elements and dust.
The gas is heated primarily by the photoelectic effect: UV radiation
is absorbed by grains, releasing electrons that share their energy
with the ambient gas. Various processes can cool the atomic gas,
depending on the temperature.
Because cooling is strongest at high temperatures ~ 10,000 K (by Lyman alpha emission from hydrogen) and low temperatures ~ 100 K (by IR radiation from carbon), atomic gas undergoes thermal instability. This causes some regions to condense, and other regions to expand. The result is a system of cold, dense clouds that are embedded in a warm, tenuous intercloud medium. These clouds are not static, but continually change shape due to pressure gradient forces on their surfaces. Using numerical simulations, the nonlinear development of thermal instability can be studied.
Since the clouds are much denser than the intercloud gas, when
gravity is included in our models, the clouds tend to sink toward the
midplane of the galactic disk. This is similar to what happens to a
rock released in air: it falls to the ground, unlike a helium ballon
is buoyant and rises up). The interstellar gas becomes vertically stratified, with dense
clouds preferentially nearer to the midplane. Some dense clouds
are present further away from the midplane, because turbulence (driven
by the magnetorotational instability) continues to stir up the whole
Evolution of a column
of interstellar gas
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This material is based upon work supported by the National Science Foundation (NSF). Any opinions, findings, conclusions, or recommendations expressed here are those of the author(s) and do not necessarily reflect the views of the NSF.