Non-thermal photons and H2 formation in the early Universe

Abstract

The cosmological recombination of H and He at z \sim 1000 and the formation of H2 during the dark ages produce a non-thermal photon excess in the Wien tail of the cosmic microwave background (CMB) blackbody spectrum. Here we compute the effect of these photons on the H- photodetachment and H2+ photodissociation processes. We discuss the implications for the chemical evolution of the Universe in the post-recombination epoch, emphasizing how important a detailed account of the full vibrational manifold of H2 and H2+ in the chemical network is. We find that the final abundances of H2, H2+, H3+ and HD are significantly smaller than in previous calculations that neglected the effect of non-thermal photons. The suppression is mainly caused by extra hydrogen recombination photons and could affect the formation rate of first stars. We provide simple analytical approximations for the relevant rate coefficients and briefly discuss the additional effect of dark matter annihilation on the considered reaction rates.