Lyα line formation in starbursting galaxies.: I.: Moderately thick, dustless, and static HI media

We investigate the Ly alpha line transfer in nearby and high-redshift starbursting galaxies, where the effects 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 a dustless, static, and uniform H I media with moderate Ly alpha line center optical depths tau (0) = 10(3)-10(6). We assume the temperatures of the media to be T = 10(1)-10(4) K, and use a Monte Carlo technique. We investigate the basic processes of the line transfer and confirm the criterion of a tau (0) > 10(3) for the validity of the diffusion approximation suggested by Neufeld in 1990, where a is the Voigt parameter. Adopting the model for evolution of a galactic supershell suggested by Tenorio-Tagle et al. in 1999, we performed calculations on the Ly alpha line formation for each evolutionary stage of an expanding supershell. The emergent Ly 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 Ly alpha emission profile from an expanding recombining supershell can be marginally detected.