Crossing walls and windows: the curious escape of Lyman-$\alpha$ photons through ionised channels

Abstract

The diverse Lyman-alpha (Ly$\alpha$) line profiles are essential probes of gas in and around galaxies. While isotropic models can successfully reproduce a range of Ly$\alpha$ observables, the correspondence between the model and actual physical parameters remains uncertain. We investigate the effect of anisotropies of Ly$\alpha$ escape using a simplified setup: a hole (fractional size $\tilde{s}$) within a semi-infinite slab with constant column density. Due to the slab's high line-centre optical depth (${\tau }_0\gtrsim 10^{5-6}$), most photons should escape through the empty channel. However, our numerical findings indicate that only a fraction $\sim \tilde{s}$ of photons exit through this channel. To explain this puzzle, we developed an analytical model describing the scattering and transmission behaviour of Ly$\alpha$ photons in an externally illuminated slab. Our findings show that the number of scatterings per reflection follows a Lévy distribution ($\propto N^{-3/2}$). This means that the mean number of scatterings is orders of magnitude greater than expectations, facilitating a shift in frequency and the subsequent photon escape. Our results imply that Ly$\alpha$ photons are more prone to traverse high-density gas and are surprisingly less biased to the `path of least resistance'. Hence, Ly$\alpha$ can trace an averaged hydrogen distribution rather than only low-column density `channels'.

7 pages, 6 figures, accepted for publication, MNRAS: Letters

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Cited by in corpus (4)

• Lyman-$\alpha$ feedback prevails at Cosmic Dawn: Implications for the first galaxies, stars, and star clusters
• Mapping reionization bubbles in the JWST era II: inferring the position and characteristic size of individual bubbles
• Lévy Flights and Leaky Boxes: Anomalous Diffusion of Cosmic Rays
• The Intrinsic Distribution of Lyman-$\alpha$ Halos