Directional Lyα equivalent boosting - I. Spherically symmetric distributions of clumps

Abstract

We quantify the directional dependence of the escape fraction of Lyman α (Lyα) and nonionizing UV-continuum photons from a multiphase medium, and investigate whether there exist directional enhancements in the Lyα equivalent width (EW). Our multiphase medium consists of spherically symmetric distributions of cold, dusty clumps embedded within a hot dust-free medium. We focus on three models from the analysis presented by Laursen et al. We find that for a Lyα and UV-continuum point source, it is possible to find an EW boost b(θ,ϕ) > 5b¯ in a few per cent of sight lines, where b¯ denotes the boost averaged over all photons. For spatially extended sources this directional dependence vanishes quickly when the size of the UV emitting region exceeds the mean distance between cold dusty clumps. Our analysis suggests that directional EW boosting can occur, and that this is mostly driven by reduced escape fractions of UV photons (which gives rise to UV-continuum 'shadows'), and less due to an enhanced Lyα escape fraction (or beaming), in certain directions.