Inferences on the distribution of Ly-alpha emission of z~7 and z~8
  galaxies
  Spectroscopic confirmation of galaxies at z~7 and above has been extremely
difficult, owing to a drop in intensity of Ly-alpha emission in comparison with
samples at z~6. This crucial finding could potentially signal the ending of
cosmic reionization. However it is based on small datasets, often incomplete
and heterogeneous in nature. We introduce a flexible Bayesian framework, useful
to interpret such evidence. Within this framework, we implement two simple
phenomenological models: a smooth one, where the distribution of Ly-alpha is
attenuated by a factor \es with respect to z~6; a patchy one where a fraction
\ep is absorbed/non-emitted while the rest is unabsorbed. From a compilation of
39 observed z~7 galaxies we find \es=0.69+-0.12 and \ep=0.66+-0.16. The models
can be used to compute fractions of emitters above any equivalent width W. For
W>25\AA, we find X^{25}_{z=7}=0.37+-0.11 (0.14+-0.06) for galaxies fainter
(brighter) than M_{UV}=-20.25 for the patchy model, consistent with previous
work, but with smaller uncertainties by virtue of our full use of the data. At
z~8 we combine new deep (5-\sigma flux limit 10^{-17}ergs^{-1}cm^{-2})
Keck-NIRSPEC observations of a bright Y-dropout identified by our BoRG Survey,
with those of three objects from the literature and find that the inference is
inconclusive. We compute predictions for future near-infrared spectroscopic
surveys and show that it is challenging but feasible to constrain the
distribution of Ly-alpha emitters at z~8 and distinguish between models.
