Extended red wings and the visibility of reionization-epoch Lyman-$\alpha$ emitters

TL;DR

This paper uses advanced simulations to show that the intrinsic Lyman-alpha emission from reionization-epoch galaxies extends further to the red than previously modeled, affecting the interpretation of galaxy visibility during reionization.

Contribution

It demonstrates that the intrinsic Lyman-alpha emission line profile is broader and more extended to the red than a Gaussian, impacting the understanding of galaxy visibility and reionization constraints.

Findings

Lyman-alpha emission extends 400-800 km/s redward, beyond Gaussian predictions.

Galaxy mergers modulate Lyman-alpha luminosity and spectral features.

Including cosmic rays and low dust reproduces observed Lyman-alpha properties.

Abstract

The visibility of the Lyman-$\alpha$ (Ly$\alpha$) emission from reionization-epoch galaxies depends sensitively on the extent of the intrinsic \lya emission redwards of 1215.67~\AA. The prominent red peak resulting from resonant radiative transfer in the interstellar medium is often modelled as a single Gaussian. We use the \textsc{Azahar} simulation suite of a massive-reionization epoch galaxy to show that a significantly larger fraction of the \lya emission extends to $400$-$800$~km~s$^{-1}$, and thus significantly further to the red than predicted by a Gaussian line profile. A cycle of frequent galaxy mergers strongly modulates the \lya luminosity, the red peak velocity and its extended red wing emerging from the galaxy, which all also strongly vary with viewing angle. The \lya emission also depends sensitively on the implemented feedback, dust and star formation physics. Our simulations including cosmic rays reproduce the observed spectral properties of reionization epoch \lya emitters (LAEs) well if we assume that the \lya emission is affected by very little dust. The visibility of LAEs can be strongly underestimated if the extended red wings of the intrinsic \lya emission are not accounted for. We discuss implications for using the visibility of LAEs to constrain the evolution of the volume-averaged neutral fraction during reionization.