Intercloud medium

Fig. 8b. Equilibrium pressure (n + ne)T and electron density. The existence of two stable regions dp/dp >0) permits two phases, F and H, to exist in pressure equilibrium. Phase F is identified with the intercloud medium, and H, with clouds. Carbon depletion lowers the density and increases the temperature of the cloud gas. The electron density in clouds is about 10 times the classical value of HI regions, 4 x 10— iiq. The value in the intercloud medium is about 2 x 10—2 cm-3...

Finally, the space not occupied by coronal gas, by HI or HII clouds, or by giant molecular clouds is occupied by the intercloud medium, consisting primarily of neutral hydrogen atoms. This "filler space occupies the second largest volume of the interstellar medium. 

The interstellar medium constitutes 10 % of the mass of the galaxy. It can be subdivided into environments with very low-density hot gas, environments with warm intercloud gas, and regions with denser and colder material (23). H and He gas are the major components of interstellar clouds molecules and submicron dust particles are only present in small concentration (22). Through gas phase reactions and solid-state chemistry, gas-grain interactions can build up complex organic molecules. Silicate and carbon-based micron-sized dust particles provide a catalytic surface for a variety of reactions when they are dispersed in dense molecular clouds 24). In cold clouds such dust particles... 

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