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The Atomic Hydrogen

The atomic phases of interstellar medium of our galaxy can be distinguished into a cold neutral medium and a warm neutral medium. A great part of the mass resides in the cold neutral medium, distributed in dense sheets or filaments (the HI clouds) occupying $ 1-4\%$ of the total volume of the Galaxy disk. These clouds, which are not gravitationally bound, have a number density of about $ 20-60$ $ cm^{-3}$ and a temperature of $ \sim 100 K$.
The warm neutral medium fills a substantial fraction ( $ \sim30-60\%$) of the available volume. This material, located mainly in photodissociation regions, has a density of about $ 0.3 -1 cm^{-3}$ and a temperature of $ \sim 6000 K$. These two phases exist in pressure balance with each other, which is a necessary condition for the existence of a two-phases medium.


Andrea Giuliani 2005-01-21