The Evolution of the Lyman-Alpha Luminosity Function During Reionization

TL;DR

This paper models the evolution of the Lyman-alpha luminosity function during reionization to constrain the timeline of hydrogen ionization in the early universe, revealing how the neutral hydrogen fraction affects galaxy visibility.

Contribution

It introduces a combined model of Lyα luminosity and UV luminosity functions to infer the IGM neutral fraction at high redshifts, emphasizing the importance of LF shape evolution.

Findings

Neutral fraction increases with redshift, reaching ~0.83 at z=7.3.

Lyα luminosity density decreases as the universe becomes more neutral.

Faint-end slope of the Lyα LF steepens with higher neutral fractions.

Abstract

The time frame in which hydrogen reionization occurred is highly uncertain, but can be constrained by observations of Lyman-alpha (Ly$\alpha$) emission from distant sources. Neutral hydrogen in the intergalactic medium (IGM) attenuates Ly$\alpha$~photons emitted by galaxies. As reionization progressed the IGM opacity decreased, increasing Ly$\alpha$~visibility. The galaxy Ly$\alpha$~luminosity function (LF) is thus a useful tool to constrain the timeline of reionization. In this work, we model the Ly$\alpha$~LF as a function of redshift, $z=5-10$, and average IGM neutral hydrogen fraction, $\overline{x}_\textsc{hi}$. We combine the Ly$\alpha$~luminosity probability distribution obtained from inhomogeneous reionization simulations with a model for the UV LF to model the Ly$\alpha$~LF. As the neutral fraction increases, the average number density of Ly$\alpha$~emitting galaxies decreases, and are less luminous, though for $\overline{x}_\textsc{hi} \lesssim 0.4$ there is only a small decrease of the Ly$\alpha$~LF. We use our model to infer the IGM neutral fraction at $z=6.6, 7.0, 7.3$ from observed Ly$\alpha$~LFs. We conclude that there is a significant increase in the neutral fraction with increasing redshift: $\overline{x}_\textsc{hi}(z=6.6)=0.08^{+ 0.08}_{- 0.05}, \, \overline{x}_\textsc{hi}(z=7.0)=0.28 \pm 0.05$ and $\overline{x}_\textsc{hi}(z=7.3)=0.83^{+ 0.06}_{- 0.07}$. We predict trends in the Ly$\alpha$~luminosity density and Schechter parameters as a function of redshift and the neutral fraction. We find that the Ly$\alpha$~luminosity density decreases as the universe becomes more neutral. Furthermore, as the neutral fraction increases, the faint-end slope of the Ly$\alpha$~LF steepens, and the characteristic Ly$\alpha$~luminosity shifts to lower values, concluding that the evolving shape of the Ly$\alpha$~LF -- not just its integral -- is an important tool to study reionization.