Luminosity functions of Lyα emitting galaxies and cosmic reionization of hydrogen

Recent observations imply that the observed number counts of Ly alpha emitters (LAEs) evolved significantly between z = 5.7 and 6.5. It has been suggested that this was due to a rapid evolution in the ionization state, and hence transmission of the intergalactic medium (IGM) which caused Ly alpha flux from z = 6.5 galaxies to be more strongly suppressed. In this paper, we consider the joint evolution of the Ly alpha and the ultraviolet luminosity functions (LFs) and show that the IGM transmission evolved between z = 6.5 and 5.7 by a factor 1.1 < R < 1.8 (95 per cent confidence level). This result is insensitive to the underlying model of the Ly alpha LF (as well as cosmic variance). Using a model for IGM transmission, we find that the evolution of the mean IGM density through cosmic expansion alone may result in a value for the ratio of transmissions as high as R = 1.3. Thus, the existing LFs do not provide evidence for overlap. Furthermore, the constraint R < 1.8 suggests that the Universe at z = 6.5 was more than half-ionized by volume, that is, x(i,V) > 0.5.