Rapid Decline of Lyman-alpha Emission Toward the Reionization Era
  The observed deficit of strongly Lyman-alpha emitting galaxies at z>6.5 is
attributed to either increasing neutral hydrogen in the intergalactic medium
(IGM) and/or to the evolving galaxy properties. To investigate this, we have
performed very deep near-IR spectroscopy of z>7 galaxies using MOSFIRE on the
Keck-I Telescope. We measure the Lyman-alpha fraction at z~8 (combined
photometric redshift peak at z=7.7) using two methods. First, we derived
NLy{\alpha}/Ntot directly using extensive simulations to correct for
incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al.
2012) that compares the z>7 galaxy spectra to models of the Lyman-alpha
equivalent width (WLy{\alpha}) distribution at z~6. We explored two simple
evolutionary scenarios: smooth evolution where Lyman-alpha is attenuated in all
galaxies by a constant factor (perhaps owing to processes from galaxy evolution
or a slowly increasing IGM opacity), and patchy evolution where Lyman-alpha is
blocked in some fraction of galaxies (perhaps due to the IGM being opaque along
only some fraction of sightlines). The Bayesian formalism places stronger
constraints compared with the direct method. Combining our data with that in
the literature we find that at z~8 the Lyman-alpha fraction has dropped by a
factor >3(84% confidence interval) using both the smooth and patchy scenarios
compared to the z~6 values. Furthermore, we find a tentative evidence that the
data favor the patchy scenario over smooth (with "positive" Bayesian evidence),
extending trends observed at z~7 to higher redshift. If this decrease is a
result of reionization as predicted by theory, then our data imply the volume
averaged neutral hydrogen fraction in the IGM to be >0.3 suggesting that the
reionization of the universe is in progress at z~8.
