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

In this paper, we consider the reprocessing of high-frequency photons emitted by He II and He I during the epoch of cosmological recombination by He I 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 similar to 40-70 per cent. Our computations imply that per helium nucleus similar to 3-6 additional photons could be produced. Therefore, a total of similar to 12-14 helium-related photons per helium atom 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 cosmic microwave background (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 He II photons on He I leads to the appearance of several additional, rather narrow spectral features in the He I recombination spectrum at low frequencies. Consequently, the signatures of helium-related features in the CMB spectral distortion from cosmological recombination at some given frequency can exceed the average level of similar to 17 per cent several times. We find that in particular the bands around nu similar to 10, similar to 35, similar to 80 and similar to 200 GHz seem to be affected strongly. In addition, we computed the changes in the cosmological ionization history, finding that only the feedback of primary He I photons on the dynamics of He II -> He I recombination has an effect, producing a change of Delta N(e)/N(e) similar to +0.17 per cent at z similar to 2300. This result seems to be similar to 2-3 times smaller than the one obtained in earlier computations for this process, however, the difference will not be very important for the analysis of future CMB data.