Directional Lya equivalent boosting I. Spherically symmetric distributions of clumps

TL;DR

This study investigates how the escape fraction and equivalent width of Ly$ ext{α}$ and UV photons depend on direction in a multiphase, spherically symmetric medium with dusty clumps, revealing conditions for directional EW boosting.

Contribution

It demonstrates that directional Ly$ ext{α}$ equivalent width boosting is possible in such media, primarily driven by UV photon escape fraction variations, extending previous models.

Findings

Directional EW boost > 5 times average occurs in a few percent of sight lines.

Extended sources show negligible directional dependence when UV emission region exceeds clump spacing.

UV shadows mainly cause the EW boosting, with less influence from Ly$ ext{α}$ beaming.

Abstract

We quantify the directional dependence of the escape fraction of Lyman-$\alpha$ (Ly$\alpha$) and non-ionising UV-continuum photons from a multiphase medium, and investigate whether there exist directional enhancements in the Ly$\alpha$ 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. (2013). We find that for a Ly$\alpha$ and UV-continuum point source, it is possible to find an EW boost $b(\theta,\phi) > 5 \bar{b}$ in a few per cent of sight lines, where $\bar{b}$ 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 Ly$\alpha$ escape fraction (or beaming), in certain directions.