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We present spectroscopic follow-up observations on two bright Ly$\alpha$emitter (LAE) candidates originally found by Krug et al. (2012) at a redshiftof z~7.7 using the Multi-Object Spectrometer for Infra-Red Exploration(MOSFIRE) at Keck. We rule out any line emission at the >5$\sigma$ level forboth objects, putting on solid ground a previous null result for one of theobjects. The limits inferred from the non-detections rule out the previousclaim of no or even reversed evolution between 5.7 < z < 7.7 in the Ly$\alpha$luminosity function (LF) and suggest a drop in the Ly$\alpha$ luminosityfunction consistent with that seen in Lyman Break galaxy (LBG) samples. Wemodel the redshift evolution of the LAE LF using the LBG UV continuum LF andthe observed rest-frame equivalent width distribution. From the comparison ofour empirical model with the observed LAE distribution, we estimate lowerlimits of the neutral hydrogen fraction to be 50-70% at z~7.7. Together withthis, we find a strong evolution in the Ly$\alpha$ optical depth characterizedby (1+z)^(2.2 $\pm$ 0.5) beyond z=6 indicative of a strong evolution of theIGM. Finally, we extrapolate the LAE LF to z~9 using our model and show that itis unlikely that large area surveys like UltraVISTA or Euclid pick up LAEs atthis redshift assuming the current depths and area.

Spectroscopic observation of Ly$\alpha$ emitters at z~7.7 and implications on re-ionization