3D Lyα radiation transfer -: II.: Fitting the Lyman break galaxy MS 1512-cB58 and implications for Lyα emission in high-z starbursts

Aims. To understand the origin of the Ly alpha line profile of the prototypical Lyman break galaxy (LBG) MS 1512-cB58 and other similar objects. To attempt a consistent fit of Ly alpha and other UV properties of this galaxy. To understand the main physical parameter(s) responsible for the large variation of Ly alpha line strengths and profiles observed in LBGs. Methods. Using our 3D Ly alpha radiation transfer code (Verhamme et al. 2006, A&A, 460, 397), we compute the radiation transfer of Ly alpha and UV continuum photons including dust. Observational constraints on the neutral gas (column density, kinematics, etc.) are taken from other analyses of this object. Results. The observed Ly alpha profile of MS 1512-cB58 is reproduced for the first time taking radiation transfer and all observational constraints into account. The observed absorption profile is found to result naturally from the observed amount of dust and the relatively high H I column density N-H. Radiation transfer effects and suppresion by dust transform a strong intrinsic Ly alpha emission with EW(Ly alpha) greater than or similar to 60 angstrom into the observed faint superposed Ly alpha emission peak. We propose that the vast majority of LBGs have intrinsically EW(Ly alpha) similar to 60-80 angstrom or larger, and that the main physical parameter responsible for the observed variety of Ly alpha strengths and profiles in LBGs is N-H and the accompanying variation of the dust content. Observed EW(Ly alpha) distributions, Ly alpha luminosity functions, and related quantities must therefore be corrected for radiation transfer and dust effects. The implications of our scenario on the duty-cycle of Ly alpha emitters are also discussed.