Cosmological production of H2 before the formation of the first galaxies

Abstract

Previous calculations of the pre-galactic chemistry have found that a small amount of H 2 , x[H 2 ] ≡ n[H 2 ]/n[H] ≈ 2.6 x 10 -6 , is produced catalytically through the H-, H 2 + and HeH + mechanisms. We revisit this standard calculation taking into account the effects of the non-thermal 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 H 2 abundance of x[H 2 ] ≈ 6 x 10 -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 H 2 + . We evaluate the heating of the diffuse pre-galactic gas from the chemical reactions that produce H 2 and from rotational transitions in H 2 , and find them to be negligible.