SPECTROSCOPIC CONFIRMATION OF z ~ 7 LYMAN BREAK GALAXIES: 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 Very Large Telescope (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 New Technology Telescope Deep Field. Both galaxies show an Lyα emission line with rest-frame equivalent widths (EWs) of the order of 15-20Å and luminosities of (2-4) x 10⁴² ergs^-1. 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 6.6 < 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 Lyα EWs (between 20 and 55Å). We conclude that the observed trend for the rising fraction of Lyα emission in Lyman break galaxies from z ~ 3 to z ~ 6 is most probably reversed from z ~ 6 to z ~ 7. Explaining the observed rapid change in the Lyα emitter fraction among the dropout 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 a set of semi-analytical models, we find that our data require a change of the neutral hydrogen fraction of the order of Δ_χHI~ 0.6 in a time Az ~ 1, provided that the escape fraction does not increase dramatically over the same redshift interval. [ABSTRACT FROM AUTHOR]