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As one stage of galaxy formation we study the gravitational contraction of a cold gas due to the radiative cooling by hydrogen molecules. Assuming that the gas consists mainly of hydrogen atoms, we consider all possible atomic and molecular processes in the gas and find that H2 molecules are formed most efficiently by way of H- formation: H+e- →H-+hν and H-+H →H2+e-. The contraction of a gas cloud is studied in the wide range of initial conditions and it is found that sufficient amounts of H2 are formed in the gas cloud so that it can evolve into a protogalaxy provided that the radiation temperature is less than 300°K. We also examine the effect of He and D on the cooling and find that their effect is negligible.
Toshiaki Hirasawa (Mon,) studied this question.