Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies
  and the epoch of reionization
  We present the final results from our ultra-deep spectroscopic campaign with
FORS2 at the ESO/VLT for the confirmation of z~7 "z--band dropout" candidates
selected from our VLT/Hawk-I imaging survey over three independent fields. In
particular we report on two newly discovered galaxies at redshift ~6.7 in the
NTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs
of the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present
the results of ultra-deep observations of a sample of i-dropout galaxies, from
which we set a solid upper limit on the fraction of interlopers. Out of the 20
z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is
systematically below the expectations drawn on the basis of lower redshift
observations: in particular there is a significant lack of objects with
intermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for
the fraction of Ly-alpha emission in LBGs that is constantly increasing from
z~3 to z~6 is most probably reversed from z~6 to z~7.
  Explaining the observed rapid change in the LAE fraction among the drop-out
population with reionization requires a fast evolution of the neutral fraction
of hydrogen in the Universe. Assuming that the Universe is completely ionized
at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we
find that our data require a change of the neutral hydrogen fraction of the
order Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape
fraction does not increase dramatically over the same redshift interval.
