Cosmological recombination: feedback of helium photons and its effect on the recombination spectrum

Abstract

In this paper we consider the re-processing of high frequency photons emitted by HeII and HeI during the epoch of cosmological recombination by HeI and HI. We demonstrate that, in comparison to computations which neglect all feedback processes, the number of cosmological recombination photons that are related to the presence of helium in the early Universe could be increased by ~40%-70%. Our computations imply that per helium nucleus ~3-6 additional photons could be produced. Therefore, a total of ~12-14 helium-related photons are emitted during cosmological recombination. This is an important addition to cosmological recombination spectrum which in the future may render it slightly easier to determine the primordial abundance of helium using differential measurements of the CMB energy spectrum. Also, since these photons are the only witnesses of the feedback process at high redshift, observing them in principle offers a way to check our understanding of the recombination physics. Here most interestingly, the feedback of HeII photons on HeI leads to the appearance of several additional, rather narrow spectral features in the HeI recombination spectrum at low frequencies. Consequently, the signatures of helium-related features in the CMB spectral distortion due to cosmological recombination at some given frequency can exceed the average level of ~17% several times. We find that in particular the bands around nu ~10GHz, ~35GHz, ~80GHz, and ~200GHz seem to be affected strongly. In addition, we computed the changes in the cosmological ionization history, finding that only the feedback of primary HeI photons on the dynamics of HeII-->HeI recombination has an effect, producing a change of DN_e/N_e ~+ 0.17% at z~2300. This result seems to be ~2-3 times smaller than the one obtained in earlier computations for this process (abridged).

28 pages, 24 figures, submitted to MNRAS

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