Time-Dependent Corrections to the Ly-alpha Escape Probability During Cosmological Hydrogen Recombination

Abstract

We consider the effects connected with the detailed radiative transfer during the epoch of cosmological recombination on the ionization history of our Universe. We focus on the escape of photons from the hydrogen Lyman-alpha resonance at redshifts 600<~ z <~ 2000, one of two key mechanisms defining the rate of cosmological recombination. We approach this problem within the standard formulation, and corrections due to two-photon interactions are deferred to another paper. As a main result we show here that within a non-stationary approach to the escape problem, the resulting correction in the free electron fraction, N_e, is about ~1.6-1.8% in the redshift range 800<~z<~1200. Therefore the discussed process results in one of the largest modifications to the ionization history close to the maximum of Thomson-visibility function at z~1100 considered so far. We prove our results both numerically and analytically, deriving the escape probability, and considering both Lyman-alpha line emission and line absorption in a way different from the Sobolev approximation. In particular, we give a detailed derivation of the Sobolev escape probability during hydrogen recombination, and explain the underlying assumptions. We then discuss the escape of photons for the case of coherent scattering in the lab frame, solving this problem analytically in the quasi-stationary approximation and also in the time-dependent case. We show here that during hydrogen recombination the Sobolev approximation for the escape probability is not valid at the level of DP/P~5-10%. This is because during recombination the ionization degree changes significantly over a characteristic time Dz/z~10%, so that at percent level accuracy the photon distribution is not evolving (abridged)

18 pages, 12 figures, accepted version

References in corpus (14)

• How well do we understand cosmological recombination?
• Lines in the Cosmic Microwave Background Spectrum from the Epoch of Cosmological Hydrogen Recombination
• Lines in the cosmic microwave background spectrum from the epoch of cosmological helium recombination
• Primordial helium recombination I: feedback, line transfer, and continuum opacity
• Cosmological hydrogen recombination: populations of the high level sub-states
• Free-bound emission from cosmological hydrogen recombination
• Two-photon transitions in hydrogen and cosmological recombination
• The effect of forbidden transitions on cosmological hydrogen and helium recombination
• Two-Photon 2s<->1s Transitions during Recombination of Hydrogen in the Universe
• Rapid HeII->HeI recombination and radiation concerned with this process
• Cosmological hydrogen recombination: Lyn line feedback and continuum escape
• Primordial helium recombination III: Thomson scattering, isotope shifts, and cumulative results
• Primordial helium recombination II: two-photon processes
• Dynamics of Primordial Hydrogen Recombination with Allowance for a Recoil for Scattering in the Ly-alpha Line