2.2. Atomic and Molecular Gas
From HI 21-cm surveys
Rao and Briggs (1993) find
HI = (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
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
H2 / HI = 0.81, and from equation (13)
