2021MNRAS.508.3697G

The decline in abundance of Lyman-α (Lyα) emitting galaxies at z >= 6 is a powerful and commonly used probe to constrain the progress of cosmic reionization. We use the CODAII simulation, which is a radiation hydrodynamic simulation featuring a box of ∼94 comoving Mpc side length, to compute the Lyα transmission properties of the intergalactic medium (IGM) at z ∼ 5.8 to 7. Our results mainly confirm previous studies, i.e. we find a declining Lyα transmission with redshift and a large sightline-to-sightline variation. However, motivated by the recent discovery of blue Lyα peaks at high redshift, we also analyse the IGM transmission on the blue side, which shows a rapid decline at z >= 6 of the blue transmission. This low transmission can be attributed not only to the presence of neutral regions but also to the residual neutral hydrogen within ionized regions, for which a density even as low as n_ HI_∼10^–9 cm^–3 (sometimes combined with kinematic effects) leads to a significantly reduced visibility. Still, we find that ∼1 per cent of sightlines towards M_1600AB ∼ -21 galaxies at z ∼ 7 are transparent enough to allow a transmission of a blue Lyα peak. We discuss our results in the context of the interpretation of observations.