Lyman α-emitting galaxies in the epoch of reionization

With a total integration time of 168 h and a narrowband filter tuned to Ly alpha emission from z = 8.8, the UltraVISTA survey has set out to find some of the most distant galaxies, on the verge of the epoch of reionization. Previous calculations of the expected number of detected Ly alpha-emitting galaxies (LAEs) at this redshift based for example on extrapolation of lower-redshift luminosity functions did not explicitly take into account the radiative transfer of Ly alpha. In this work we have combined a theoretical model for the halo mass function, that is, the expected number of haloes per volume, with numerical results from high-resolution cosmological hydro-simulations post-processed with radiative transfer of ionizing UV and Ly alpha radiation, assessing the visibility of LAEs residing in these haloes. We have taken into account uncertainties such as cosmic variance and the anisotropic escape of Ly alpha, and predict that once the survey has finished, the probabilities of detecting none, one, or more than one are roughly 90%, 10%, and 1%, respectively. This is a significantly smaller success rate than in earlier predictions, due to the combined effect of a highly neutral intergalactic medium (IGM) scattering Ly alpha to such large distances from the galaxy that they fall outside the observational aperture, and to the actual depth of the survey being less than predicted. Because the IGM affects narrowband (NB) and broadband (BB) magnitudes differently, we argue for a relaxed colour selection criterion of m(NB) - m(BB) similar or equal to +0.85 in the AB system. Since the flux is dominated by the continuum, however, even if a galaxy is detectable in the NB, its probability of being selected as a narrowband excess object is less than or similar to 35%. Various additional properties of galaxies at this redshift are predicted, for example, the Ly alpha and UV luminosity functions, the stellar mass-halo mass relation, the spectral shape, the optimal aperture, as well as the anisotropic escape of Ly alpha through both the dusty, interstellar medium and through the partly neutral IGM. Finally, we describe and make public a fast numerical code for adding numbers with asymmetric uncertainties ("x(-sigma)-(+sigma+)") which proves significantly more precise than the standard, but wrong, way of separately adding upper and lower uncertainties in quadrature.