The Evolution of the Lyman-alpha Luminosity Function during Reionization

Abstract

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