The Evolution of the Lyman-Alpha Luminosity Function During Reionization

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, $\overline{x}_{HI}$. 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 $\overline{x}_{HI} \lesssim 0.4$ 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: $\overline{x}_{HI}(z=6.6)=0.08^{+ 0.08}_{- 0.05}, \, \overline{x}_{HI}(z=7.0)=0.28 \pm 0.05$ and $\overline{x}_{HI}(z=7.3)=0.69 \pm 0.11$. 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.