Cosmological production of H2 before the formation of the first galaxies
Abstract
Previous calculations of the pregalactic chemistry have found that a small amount of H2, x[H2]=n[H2]/n[H] = 2.6e-6, is produced catalytically through the H-, H2+, and HeH+ mechanisms. We revisit this standard calculation taking into account the effects of the nonthermal radiation background produced by cosmic hydrogen recombination, which is particularly effective at destroying H- via photodetachment. We also take into consideration the non-equilibrium level populations of H2+, which occur since transitions among the rotational-vibrational levels are slow compared to photodissociation. The new calculation predicts a final H2 abundance of x[H2] = 6e-7 for the standard cosmology. This production is due almost entirely to the H- mechanism, with ~1 per cent coming from HeH+ and ~0.004 per cent from H2+. We evaluate the heating of the diffuse pregalactic gas from the chemical reactions that produce H2 and from rotational transitions in H2, and find them to be negligible.
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