Cosmological production of H2 before the formation of the first galaxies

Previous calculations of the pregalactic chemistry have found that a small amount of H2, x[H2] ≡ n[H2]/n[H] ≈ 2.6 × 10 , is produced catalytically through the H−, H+2 , 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 H+2 , 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] ≈ 6 × 10 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 H+2 . 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.