Properties of Atomic Gas in Spiral Galaxies

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(Abridged) Although both Warm (WNM, 10⁴ K) and Cool (CNM, about 100 K) atomic phases coexist in many environments, the dominant mass contribution within a galaxy's star-forming disk (R₂5) is that of the CNM. Mass fractions of 60 to 90% are found within R₂5, in the form of moderately opaque filaments with a face-on surface covering factor 15%. The kinetic temperature of the CNM increases systematically with galactocentric radius, from some 50 to 200 K, as expected for a radially declining thermal pressure in the galaxy mid-plane. Galaxies of different Hubble type form a nested distribution in Tₖ(R), apparently due to the mean differences in pressure which result from the different stellar and gas surface densities. The opaque CNM disappears abruptly near R₂5, where the low thermal pressure can no longer support the condensed atomic phase. The CNM is apparently a prerequisite for star formation. Median line profiles of the CNM display an extremely narrow line core (FWHM about 6 km/s) together with broad Lorentzian wings (FWHM about 30 km/s). The line core is consistent with only opacity broadening of a thermal profile. The spatial distribution of CNM linewidths is not random. High linewidths occur in distinct shell-like structures with diameter of 100's of pc to kpc's, which show some correlation with diffuse H-alpha shells. The primary source of ``turbulent'' linewidth in the atomic ISM appears to be organized motions due to localized energy injection on a scale of a few 100 pc.