ASYMMETRIC ABSORPTION PROFILES OF Lyα AND Lyβ IN DAMPED Lyα SYSTEMS

Damped Lyα systems observed in the quasar spectra are characterized by a high neutral hydrogen column density, . The absorption wing profiles are often fitted using the Voigt function due to the fact that the scattering cross section near the resonant line center is approximately described by the Lorentzian function. Since a hydrogen atom has infinitely many p states that participate in the electric dipole interaction, the cross section starts to deviate from the Lorentzian in an asymmetric way in the line wing regions. We investigate this asymmetry in the absorption line profiles around Lyα and Lyβ as a function of the neutral hydrogen column density . In terms of Δλ ≡ λ − λ α , we expand the Kramers–Heisenberg formula around Lyα to find σ(λ) ≃ (0.5 f 12 ) 2 σ T (Δλ/λ α ) −2 [1 + 3.792(Δλ/λ α )], where f 12 and σ T are the oscillator strength of Lyα and the Thomson scattering cross section, respectively. In terms of Δλ 2 ≡ λ − λ β in the vicinity of Lyβ, the total scattering cross section, given as the sum of cross sections for Rayleigh and Raman scattering, is shown to be σ(λ) ≃ σ T (0.5 f 13 ) 2 (1 + R 0 )(Δλ 2 /λ β ) −2 [1 − 24.68(Δλ 2 /λ β )] with f 13 and the factor R 0 = 0.1342 being the oscillator strength for Lyβ and the ratio of the Raman cross section to Rayleigh cross section, respectively. A redward asymmetry develops around Lyα, whereas a blue asymmetry is obtained for Lyβ. The absorption center shifts are found to be almost proportional to the neutral hydrogen column density.