From mirrors to windows: Lyman-alpha radiative transfer in a very clumpy medium

Lyman-alpha (Lyα) is the strongest emission line in the universe and is frequently used to detect and study the most distant galaxies. Because Lyα is a resonant line, photons typically scatter prior to escaping; this scattering process complicates the interpretation of Lyα spectra, but also encodes a wealth of information about the structure and kinematics of neutral gas in the Galaxy. Modeling the Lyα line therefore allows us to study tiny-scale features of the gas. Curiously, observed Lyα spectra can be modeled successfully with very simple, homogeneous geometries (such as an expanding, spherical shell), whereas more realistic, multiphase geometries often fail to reproduce the observed spectra. This seems paradoxical since the gas in galaxies is known to be multiphase. In this Letter, we show that spectra emerging from clumpy geometries with a large number ($\gtrsim 10$ for a clump column density of $N_{HI,cl}\sim {10}^{17}{cm}^{-2}$) of clouds along the line of sight converge to the predictions from simplified, homogeneous models. We suggest that this resolves the apparent discrepancy and may provide a way to study the gas structure in galaxies on scales far smaller than can be probed in either cosmological simulations or direct (i.e., spatially resolved) observations.