Origin of Lyα absorption in nearby starbursts and implications for other galaxies

Context. Despite the privileged position that Lyman-alpha (Ly alpha) emission line holds in the exploration of the distant universe and modern observational cosmology, the origin of the observed diversity of Ly alpha profiles still needs to be thoroughly explained. Observations of nearby star forming galaxies bring their batch of apparent contradictions between Ly alpha emission and their physical parameters, and call for a detailed understanding of the physical processes at work. One of the most metal-poor galaxies known, IZw 18, is of particular interest in this context. Aims. We fit the Ly alpha spectrum of IZw 18 to understand the origin of the damped absorption profile and its spatial variations across the NW region, and establish a general picture of the physical parameters governing the Ly alpha strength and profile both in local and in high-z galaxies. Methods. We used a 3D Ly alpha radiation transfer code to model Hubble Space Telescope (HST) observations of IZw 18. Observational constraints of relevant parameters such as dust or HI column density were derived from previous studies and from the present analysis. Different geometrical configurations of the source and the neutral gas were explored. Results. The integrated Ly alpha profile of NWregion of IZw 18 is reproduced using the observed small amount of dust (E(B-V) approximate to 0.05) and a spherical HI shell with N(HI) = 6.5 x 10(21) cm(-2). Such a high column density makes it possible to transform a strong Ly alpha emission (EW(Ly alpha) = 60 angstrom) into a damped absorption even with low extinction. When a slab geometry is applied and a given line of sight is chosen, the Ly alpha profile can be successfully reproduced with no dust at all and N(HI) = 3 x 10(21) cm(-2). The spatial variations of the profile shape are naturally explained by radiation transfer effects, i.e. by scattering of Ly alpha photons, when the observed surface brightness profile of the source is taken into account. In the case of outflowing inter stellar medium (ISM), as commonly observed in Lyman break galaxies (LBGs), a high N(HI) and dust content are required to observe Lya in absorption. For nearly static neutral gas as observed in IZw 18 and other low-luminosity galaxies, only a small amount of dust is required provided a sufficiently high N(HI) covers the galaxy. We also show how geometrical and apertures effects affect the Ly alpha profile.