Properties of Atomic Gas in Spiral Galaxies

Summary

Physical properties of the atomic gas in spiral galaxies are briefly considered. Although both Warm (WNM, 10⁴ K) and Cool (CNM, ∼ 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. The CNM is concentrated within moderately opaque filaments with a face-on surface covering factor of about 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 T_K(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. Although difficult to prove, all indications are that at least the outer disk and possibly the inter-arm atomic gas are in the form of WNM, which accounts for about 50% of the global total. Median line profiles of the CNM display an extremely narrow line core (FWHM ∼ 6 km s⁻¹) together with broad Lorentzian wings (FWHM ∼ 30 km s⁻¹). The line core is consistent with only opacity broadening of a thermal profile.