Turbulence and Structure in the Diffuse Interstellar Medium

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. 

Development of thermal instability in interstellar gas


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.
Later evolution of clouds

The interstellar medium is subject to other instabilities, in addition to the thermal instability that creates clouds.  One of these instabilities is called the magnetorotational instability (MRI) .  The MRI arises due to interaction between magnetic tension forces and sheared rotation in a galaxy (since the interior of the galaxy rotates more rapidly than the exterior).  As a consequence of MRI, the interstellar medium becomes highly turbulent.  Shear stretches out individual clouds, and turbulence both breaks up clouds and causes cloud agglomeration.  The interstellar medium is constantly being stirred up by recurring episodes of MRI, even far from any stars. 

Development of thermal and magnetorotational instability in 3D model of a local interstellar region

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 (which 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 system.

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.