Lyα Line Formation in Starbursting Galaxies. I. Moderately Thick, Dustless, and Static HiMedia

We investigate the Lyα 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 H I media with moderate optical depths τ0 = 10 3−7, where τ0 is the line center optical depth of Lyα and temperatures T = 101−4 K using a Monte Carlo code. We investigate the basic physics of the line transfer and confirm the criterion of aτ0 103 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 Lyα line formation for each evolutionary stage of an expanding supershell. The emergent Lyα 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. Subject headings: line: formation — radiative transfer — galaxies: starburst — galaxy: formation 1.