The Atomic Component

Abstract

We review the physical conditions and the distribution of the three phases which consitute the atomic component: the cold clouds, the warm, neutral medium and the warm, ionized medium. Together, these three phases occupy about ≳ 40% of the interstellar volume and contain half the interstellar mass. The size of the H I disk is comparable or exceeds that of the stellar disk. In the outer Galaxy, a spiral pattern is clearly discernable. We discuss extensively the distribution of the three phases near the sun as well as the physics and the limitations of various probes that have been used to study the atomic component. The ionization of the phases is studied in depth and we conclude that most of the interstellar electrons reside in the warm, ionized medium. We review various determinations of interstellar pressure and its scale height. Finally, we derive the filling factors of the atomic phases as a function of z. [Unless otherwise stated, R₀ = 10 kpc]

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