Can the intergalactic medium cause a rapid drop in Lyα emission at z > 6?

The large cross-section of the Lyα line makes it a sensitive probe of the ionization state of the intergalactic medium (IGM). Here, we present the most complete study to date of the IGM Lyα opacity, and its application to the redshift evolution of the 'Lyα fraction', i.e. the fraction of colour-selected galaxies with a detectable Lyα emission line. We use a tiered approach, which combines large-scale seminumeric simulations of reionization with moderate-scale hydrodynamic simulations of the ionized IGM. This allows us to simultaneously account for evolution in both: (i) the opacity from an incomplete (patchy) reionization, parametrized by the filling factor of ionized regions, QH II; and (ii) the opacity from self-shielded systems in the ionized IGM, parametrized by the average photoionization rate inside H II regions, 〈Γ12〉H II. In contrast to recent empirical models, attenuation from patchy reionization has a unimodal distribution along different sightlines, while attenuation from self-shielded systems is more bimodal. We quantify the average IGM transmission in our (QH II, 〈Γ12〉H II) parameter space, which can easily be used to interpret new data sets. Our new, improved models highly disfavour an evolution in ∠Γ12〉H II as the sole driver of a large change in IGM opacity. Using current observations, we predict that the Lyα fraction cannot drop by more than a factor of ≈2 with IGM attenuation alone, even for H II filling factors as low as QH II ≳ 0.1. Larger changes in the Lyα fraction could result from a co-evolution with galaxy properties. Marginalizing over 〈Γ12〉H II, we find that current observations constrain QH II(z ≈ 7) ≤ 0.6, at a 68 per cent confidence level (CL). However, all of our parameter space is consistent with observations at 95 per cent CL, highlighting the need for larger observational samples at z ≥ 6.