The effect of the intergalactic environment on the observability of Lyα emitters during reionization

Observations of high-redshift Ly alpha sources are a major tool for studying the high-redshift universe and are one of the most promising ways to constrain the later stages of reionization. The understanding and interpretation of the data is far from straightforward, however. We discuss the effect of the reionizing intergalactic medium (IGM) on the observability of Ly alpha sources based on large simulations of early structure formation with radiative transfer. This takes into account self-consistently the reionization history, density, velocity and ionization structures and non-linear source clustering. We find that all fields are highly anisotropic and as a consequence there are very large variations in opacity among the different lines of sight. The velocity effects, from both infall and source peculiar velocity are most important for the luminous sources, affecting the line profile and depressing the bright end of the luminosity function. The line profiles are generally asymmetric and the line centres of the luminous sources are always absorbed due to the high density of the local IGM. For both luminous and average sources the damping wing effects are of similar magnitude and remain significant until fairly late, when the IGM is ionized between 30 and 70 per cent by mass. The ionizing flux in the ionized patch surrounding a high-density peak is generally strongly dominated, particularly at late times, by the cluster of faint sources, rather than the central massive galaxy. Our results reproduce well the observed mean opacity of the IGM at z similar to 6. The IGM absorption does not change appreciably the correlation function of sources at high redshift. Our derived luminosity function assuming constant mass-to-light ratio provides an excellent match to the shape of the observed luminosity function at z = 6.6 with faint-end slope of alpha = -1.5. The resulting mass-to-light ratio implies that the majority of sources responsible for reionization are too faint to be observed by the current surveys.