# Resonant line transfer in a fog: Using Lyman-alpha to probe tiny structures in atomic gas

## TL;DR

This study investigates how Lyman-alpha radiative transfer behaves in highly clumpy, small-scale neutral gas environments, revealing that at high covering factors, such media appear homogeneous in their spectral signatures.

## Contribution

It demonstrates that a large number of neutral gas clumps can mimic a homogeneous medium in Lyman-alpha spectra, explaining observational fits and simulation limitations.

## Key findings

- High covering factors lead to spectra similar to homogeneous models.
- Small-scale structure causes underestimation of neutral gas coverage in simulations.
- Recurrent modeling issues are linked to unresolved small-scale gas clumps.

## Abstract

Motivated by observational and theoretical work which both suggest very small scale ($\lesssim 1\,$pc) structure in the circum-galactic medium of galaxies and in other environments, we study Lyman-$\alpha$ (Ly$\alpha$) radiative transfer in an extremely clumpy medium with many "clouds" of neutral gas along the line of sight. While previous studies have typically considered radiative transfer through sightlines intercepting $\lesssim 10$ clumps, we explore the limit of a very large number of clumps per sightline (up to $f_{\mathrm{c}} \sim 1000$). Our main finding is that, for covering factors greater than some critical threshold, a multiphase medium behaves similar to a homogeneous medium in terms of the emergent Ly$\alpha$ spectrum. The value of this threshold depends on both the clump column density and on the movement of the clumps. We estimate this threshold analytically and compare our findings to radiative transfer simulations with a range of covering factors, clump column densities, radii, and motions. Our results suggest that (i) the success in fitting observed Ly$\alpha$ spectra using homogeneous "shell models" (and the corresponding failure of multiphase models) hints towards the presence of very small-scale structure in neutral gas, in agreement within a number of other observations; and (ii) the recurrent problems of reproducing realistic line profiles from hydrodynamical simulations may be due to their inability to resolve small-scale structure, which causes simulations to underestimate the effective covering factor of neutral gas clouds.