Primordial helium recombination. I. Feedback, line transfer, and continuum opacity

Precision measurements of the cosmic microwave background temperature anisotropy on scales l > 500 will be available in the near future. Successful interpretation of these data is dependent on a detailed understanding of the damping tail and cosmological recombination of both hydrogen and helium. This paper and two companion papers are devoted to a precise calculation of helium recombination. We discuss several aspects of the standard recombination picture, and then include feedback, radiative transfer in He I lines with partial redistribution, and continuum opacity from H I photoionization. In agreement with past calculations, we find that He II recombination proceeds in Saha equilibrium, whereas He I recombination is delayed relative to Saha due to the low rates connecting excited states of He I to the ground state. However, we find that at z < 2200 the continuum absorption by the rapidly increasing H I population becomes effective at destroying photons in the He I 2(1)P(o) - 1(1)S line, causing He I recombination to finish around z approximate to 1800, much earlier than previously estimated.