SPECTROSCOPIC CONFIRMATION OF<i>z</i>∼ 7 LYMAN BREAK GALAXIES: PROBING THE EARLIEST GALAXIES AND THE EPOCH OF REIONIZATION

Abstract

We present the final results from our ultra-deep spectroscopic campaign with\nFORS2 at the ESO/VLT for the confirmation of z~7 "z--band dropout" candidates\nselected from our VLT/Hawk-I imaging survey over three independent fields. In\nparticular we report on two newly discovered galaxies at redshift ~6.7 in the\nNTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs\nof the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present\nthe results of ultra-deep observations of a sample of i-dropout galaxies, from\nwhich we set a solid upper limit on the fraction of interlopers. Out of the 20\nz-dropouts observed we confirm 5 galaxies at 6.7 &lt; z &lt; 7.1. This is\nsystematically below the expectations drawn on the basis of lower redshift\nobservations: in particular there is a significant lack of objects with\nintermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for\nthe fraction of Ly-alpha emission in LBGs that is constantly increasing from\nz~3 to z~6 is most probably reversed from z~6 to z~7.\n Explaining the observed rapid change in the LAE fraction among the drop-out\npopulation with reionization requires a fast evolution of the neutral fraction\nof hydrogen in the Universe. Assuming that the Universe is completely ionized\nat z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we\nfind that our data require a change of the neutral hydrogen fraction of the\norder Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape\nfraction does not increase dramatically over the same redshift interval.\n