Journal Article

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

J. Chluba and R. A. Sunyaev

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 402, issue 2, pages 1221-1248
Published in print February 2010 | ISSN: 0035-8711
Published online February 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.15959.x
Cosmological recombination: feedback of helium photons and its effect on the recombination spectrum

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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 H i. 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 per cent. Our computations imply that per helium nucleus ∼3–6 additional photons could be produced. Therefore, a total of ∼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 ∼17 per cent several times. We find that in particular the bands around ν∼ 10, ∼35, ∼80 and ∼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 ΔNe/Ne∼+0.17 per cent at z∼ 2300. This result seems to be ∼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.

Keywords: atomic processes; radiative transfer; cosmic microwave background; cosmology: theory

Journal Article.  21575 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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