Lyα-emitting galaxies as a probe of reionization: large-scale bubble morphology and small-scale absorbers

The visibility of Ly alpha-emitting galaxies during the Epoch of Reionization is controlled by both diffuse HI patches in large-scale bubble morphology and small-scale absorbers. To investigate their impacts on Ly alpha transfer, we apply a novel combination of analytic modelling and cosmological hydrodynamical, radiative transfer simulations to three reionization models: (i) the 'bubble' model, where only diffuse HI outside ionized bubbles is present; (ii) the 'web' model, where HI exists only in overdense self-shielded gas; and (iii) the hybrid 'web-bubble' model. The three models can explain the observed Ly alpha luminosity function equally well, but with very different HI fractions. This confirms a degeneracy between the ionization topology of the intergalactic medium (IGM) and the HI fraction inferred from Ly alpha surveys. We highlight the importance of the clustering of small-scale absorbers around galaxies. A combined analysis of the Ly alpha luminosity function and the Ly alpha fraction can break this degeneracy and provide constraints on the reionization history and its topology. Constraints can be improved by analysing the full MUV-dependent redshift evolution of the Ly alpha fraction of Lyman break galaxies. We find that the IGM-transmission probability distribution function is unimodal for bubble models and bimodal in web models. Comparing our models to observations, we infer that the neutral fraction at z similar to 7 is likely to be of the order of tens of per cent when interpreted with bubble or web-bubble models, with a conservative lower limit similar to 1 per cent when interpreted with web models.