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 similar or equal to 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 similar to 6.7 in the New Technology Telescope Deep Field. Both galaxies show an Ly alpha emission line with rest-frame equivalent widths (EWs) of the order of 15-20 angstrom and luminosities of (2-4) x 10(42) erg s(-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 Lya EWs (between 20 and 55 angstrom). We conclude that the observed trend for the rising fraction of Ly alpha emission in Lyman break galaxies from z similar to 3 to z similar to 6 is most probably reversed from z similar to 6 to z similar to 7. Explaining the observed rapid change in the Ly alpha 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 Delta chi(HI) similar to 0.6 in a time Delta z similar to 1, provided that the escape fraction does not increase dramatically over the same redshift interval.