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2.2. Atomic and Molecular Gas

From HI 21-cm surveys Rao and Briggs (1993) find OmegaHI = (1.88 ± 0.45) x 10-4h-1 (see also Fall & Pei 1993, Zwaan et al. 1998). A 32% addition (for 24% helium mass fraction) to the Rao-Briggs value for helium yields

Equation 13 (13)

This result is quite secure because absorption line studies show directly that most of the HI is in systems with column densities high enough to detect in 21-cm emission. These columns are also generally high enough to resist photoionization so we are counting here mostly regions dominated by neutral atomic gas.

The ratios H2 / HI of mass in molecular and atomic hydrogen listed in Table 1 for each morphological type are median values taken from the compilation of Young & Scoville (1991), who base the molecular hydrogen masses on CO observations. The representative masses in atomic hydrogen in Table 1 are from Roberts & Haynes (1994). Combining these numbers with the distribution µt in morphology, weighted by HI mass, we obtain the global ratio in galaxies rhoH2 / rhoHI = 0.81, and from equation (13)

Equation 14 (14)