Gas, dust and molecules in the Galaxy

The relative abundances of cool neutral hydrogen, carbon monoxide and formaldehyde are studied using all the available observational data in the literature. The obtained mean values Nsub(H I)/Nsub(H2CO), Nsub(H I)/Nsub(CO), Nsub(CO/Nsub(H2CO) are approximately constant in the dark clouds of the solar neighbourhood and in the distant molecular clouds. The observed correlations Nsub(CO), Asub(γ) and Nsub(H2CO), Asub(γ) show that formaldehyde can also be used as an indicator of molecular hydrogen. The ratio Nsub(H I)/Asub(γ) depends on densities and decays considerably in the ranges of visual absorptions in which the molecules become detectable (Asub(γ) > approximately 2 mg); an average of Nsub(H2)/Nsub(H I) approximately 10 is calculated for the dense dark clouds. Indications of systematic temperature gradients deltaT/deltaAsub(γ) are found for formaldehyde and neutral hydrogen inside the dark clouds, and qualitative comparisons are made with theoretical quantum mechanics calculations. (Auth.)