High-sensitivity observations of the 2.6-mm spectral line of (C-12)O show the detailed distribution of cold interstellar material along the galactic equator in the first longitude quadrant. The fact that (C-12)O and (C-13)O lines have the same general shape indicates effective optical thinness of the (C-12)O lines and thus permits derivation of the galactic distribution of CO volume and surface densities. Relating the CO densities to those of H2 on the basis of solar abundances gives the galactic arrangement of H2, and, with the distribution of H I, of total interstellar nucleons smoothed over galactic azimuth. The mode of the CO and H2 volume-density distributions is 5.8 kpc; here the volume densities of CO and H2 are respectively 0.00012 and 2.0 per cu cm, and the surface densities projected through the galactic disk are respectively 0.01 and 12 solar masses per sq pc. Integration of the distributions gives the galactic mass of CO as 2 million solar masses and of H2 as 2 billion solar masses. At galactic radii greater than 5 kpc, the surface density of gas is approximately 4 per cent of the surface density of total mass predicted by Innanen (1973), implying a uniform efficiency for star formation.
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Gordon et al. (1976) studied this question.