Accurate Lyα scattering cross-section and red damping wing in the reionization epoch
Abstract
By combining a detailed computation of the Lyα scattering cross-section with a standard Λ-cold dark matter cosmology, we investigate the accurate intergalactic absorption profiles of neutral hydrogen. Various Lyα absorption systems in the spectra of distant quasars provide crucial measurements of the physical environment of the intergalactic medium. Recent high-redshift observations and cosmological hydrodynamic simulations consistently indicate that the universe has been reionized at z ∼ 6, where a significant flux drop around the Lyα absorption is detected. Since the Lyα opacity is highly sensitive to the presence of atomic hydrogen, the line centre is easily saturated with a tiny neutral fraction (nHI/nH ∼ 10-4). Furthermore, the column density of neutral hydrogen is expected to exceed NHI ∼ 1021 cm-2 through the reionization epoch, which implies that contributions of the off-centre scattering also become important. In order to accurately describe radiative damping phenomena, we consider a fully quantum mechanical scattering cross-section based on the second-order time-dependent perturbation theory. We quantitatively compare our scattering scheme with classical scattering theories, such as the Lorentzian function and the two-level approximation introduced by Peebles. Neglecting contributions of all possible intermediate states, the two-level approximation seriously underestimates scattering probabilities for non-resonant scattering. Adopting the recent concordance cosmology, the Gunn-Peterson absorption profiles are calculated. We find that when the neutral column density reaches NHI ∼ 1021 cm-2, the Peebles approximation yields about 10 per cent overestimation of the transmitted flux and about 5 per cent narrower half-width at half-maximum. For practical applications, we also introduce an analytic function that corrects the classical approximations with an excellent accuracy.
- Publication:
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Monthly Notices of the Royal Astronomical Society
- Pub Date:
- January 2015
- DOI:
- 10.1093/mnras/stu2030
- Bibcode:
- 2015MNRAS.446..264B
- Keywords:
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- line: profiles;
- scattering;
- galaxies: high-redshift;
- intergalactic medium;
- cosmology: theory;
- dark ages;
- reionization;
- first stars