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

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 optical depths \tau_0=10^{3-7}, where \tau_0 is the line center optical depth of Lyman alpha and temperatures T=10^{1-4}K using a Monte Carlo code. We investigate the basic physics of the line transfer and confirm the criterion of a\tau_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. in 1999, we performed a detailed calculation 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. It is found that the absorption troughs expected in most of evolutionary stages are not wide enough to be observed with current instruments, whereas the trough in the emission from an expanding recombining supershell can be marginally detected.