Lyman alpha line formation in starbursting galaxies I. Moderately thick, dustless, and static HI media

Abstract

We investigate the Lyman alpha line transfer in nearby and high redshift starbursting galaxies, where the effect of high optical depths and the role of dust in the scattering medium are expected to be conspicuous and should be treated in a very careful manner. We present our first results in dustless, static, and uniform HI media with moderate Lyman alpha line center optical depths τ_0=10^{3-6}. We assume that the temperatures of media to be T=10^{1-4}K, and use a Monte Carlo technique. We investigate the basic processes of the line transfer and confirm the criterion of aτ_0>10^3 for the validity of diffusion approximation suggested by Neufeld in 1990, where $a$ is the Voigt parameter. Adopting the model suggested by Tenorio-Tagle et al., we performed calculations on the Lyman alpha line formation for each evolutionary stage of an expanding supershell. The emergent Lyman alpha profiles are characterized by the double peaks and the absorption trough at the line center. We found that the absorption troughs expected in most of the evolutionary stages are not wide enough to be observed with current instruments. However, the absorption trough in the Lyman alpha emission profile from an expanding recombining supershell can be marginally detected.