SIMBAD references

We investigate the asymmetry of the scattering cross section of radiation around Lyα by atomic hydrogen, which may be applied to analyses of scattering media with high neutral hydrogen column densities, including damped Lyα absorption systems of quasars. The exact scattering cross section is given by the Kramers-Heisenberg formula obtained from fully quantum mechanical second-order time-dependent theory, where, in the case of hydrogen, each matrix element is given in a closed analytical form. The asymmetric deviation of the scattering cross section from the Lorentzian near the line center is computed by expanding the Kramers-Heisenberg formula in terms of Δω/ω_Lyα, where ω_Lyα is the angular frequency of the Lyα transition and Δω is the deviation of incident radiation from ω_Lyα. To the first order of Δω/ω_Lyα, we obtain σ(ω)=σ_T(0.5f₁₂ω_Lyα/Δω)²(1-1.79Δω/ω_Lyα), where σ_T is the Thomson scattering cross section and f₁₂=0.4162 is the oscillator strength for the Lyα transition. With this deviation, the line center of the damped wing profile apparently shifts blueward of the true Lyα line center. In the case of a damped Lyα system with an H I column density 5x10²¹ cm^–2, the apparent line-center shift relative to the true center amounts to 0.2 Å, resulting in an underestimation of the redshift by Δz∼10^–4. A measurable underestimation by an amount Δz∼10^–3 is expected for absorbing systems with N_HI_≥4x10²² cm^–2.