THE LACK OF INTENSE Lyα IN ULTRADEEP SPECTRA OF z=7 CANDIDATES IN GOODS-S: IMPRINT OF REIONIZATION?

We present ultradeep optical spectroscopy obtained with FORS2 on the Very Large Telescope (VLT) of seven Lyman-break galaxy (LBG) candidates at z > 6.5 selected in the GOODS-S field from Hawk-I/VLT and WFC3/HST imaging. For one galaxy we detect a low significance emission line (S/N <= 7), located at lambda = 9691.5 +/- 0.5 angstrom and with flux 3.4 x 10(-18) erg cm(-2) s(-1). If identified as Ly alpha, it places the LBG at redshift z = 6.972 +/- 0.002, with a rest-frame equivalent width EWrf = 13 angstrom. Using Monte Carlo simulations and conservative EW distribution functions at 2 < z < 6, we estimate that the probability of observing no galaxies in our data with S/N > 10 is similar or equal to 2%, and that of observing only one galaxy out of seven with S/N = 5 is similar or equal to 4%, but these can be as small as similar to 10(-3), depending on the details of the EW distribution. We conclude that either a significant fraction of the candidates is not at high redshift or that some physical mechanism quenches the Lya emission emerging from the galaxies at z > 6.5, abruptly reversing the trend of the increasing fraction of strong emitters with increasing redshift observed up to z similar to 6.5. We discuss the possibility that an increasingly neutral intergalactic medium is responsible for such quenching.