2025MNRAS.542..762Y

The visibility of the Lyman-$\alpha$ (Ly $\alpha$) emission from reionization-epoch galaxies depends sensitively on the extent of the intrinsic Ly $\alpha$ emission redwards of 1215.67 Å. The prominent red peak resulting from resonant radiative transfer in the interstellar medium is often modelled as a single Gaussian. We use the AZAHAR simulation suite of a massive-reionization-epoch galaxy to show that a significantly larger fraction of the Ly $\alpha$ 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 Ly $\alpha$ luminosity, the red peak velocity and its extended red wing emerging from the galaxy, which all also strongly vary with viewing angle. The Ly $\alpha$ 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 Ly $\alpha$ emitters (LAEs) well if we assume that the Ly $\alpha$ emission is affected by very little dust. The visibility of LAEs can be strongly underestimated if the extended red wings of the intrinsic Ly $\alpha$ 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.