Asymmetric Absorption Profiles of Lyα and Lyβ in Damped Lyman Alpha Systems

Abstract

Damped Lyα systems (DLAs) observed in the quasar spectra are characterized by a high neutral hydrogen column density $N_{HI}>2\times 10^{20}\mathrm{c}{\mathrm{m}}^{-2}$. The absorption wing profiles are often fitted using the Voigt function due to the fact that the scattering cross section near 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 $N_{HI}$. In terms of $\Delta \lambda \equiv \lambda -{\lambda }_{\alpha }$ we expand the Kramers-Heisenberg formula around Lyα to find $\sigma (\lambda )\simeq (0.5f_{12}{)}^2{\sigma }_T(\Delta \lambda /{\lambda }_{\alpha }{)}^{-2}[1+3.792(\Delta \lambda /{\lambda }_{\alpha })]$, where $f_{12}$ and ${\sigma }_T$ are the oscillator strength of Ly$\alpha$ and the Thomson scattering cross section, respectively. In terms of $\Delta {\lambda }_2\equiv \lambda -{\lambda }_{\beta }$ 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 $\sigma (\lambda )\simeq {\sigma }_T(0.5f_{13}{)}^2(1+R_0)(\Delta {\lambda }_2/{\lambda }_{\beta }{)}^{-2}[1-24.68(\Delta {\lambda }_2/{\lambda }_{\beta })]$ with $f_{13}$ and the factor $R_0=0.1342$ being the oscillator strength for Lyβ and the ratio of 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.

30 pages, 8 figures, 3 tables, accpeted for publication in ApJ

References in corpus (3)

• A luminous quasar at a redshift of z = 7.085
• Metallicity Evolution of Damped Lyman-alpha Systems out to z~5
• The First Positive Detection of Molecular Gas in a GRB Host Galaxy

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