Lyman- transmission properties of the intergalactic medium in the CoDaII simulation

The decline in abundance of Lyman-alpha (Ly alpha) emitting galaxies at z greater than or similar to 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 similar to 94 comoving Mpc side length, to compute the Ly alpha transmission properties of the intergalactic medium (IGM) at z similar to 5.8 to 7. Our results mainly confirm previous studies, i.e. we find a declining Ly alpha transmission with redshift and a large sightline-to-sightline variation. However, motivated by the recent discovery of blue Ly alpha peaks at high redshift, we also analyse the IGM transmission on the blue side, which shows a rapid decline at z greater than or similar to 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)similar to 10(-9) cm(-3) (sometimes combined with kinematic effects) leads to a significantly reduced visibility. Still, we find that similar to 1 per cent of sightlines towards M-1600AB similar to -21 galaxies at z similar to 7 are transparent enough to allow a transmission of a blue Ly alpha peak. We discuss our results in the context of the interpretation of observations.