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 λ = 9691.5 ± 0.5 Å and with flux 3.4 × 10 −18 erg cm −2 s −1 . If identified as Lyα, it places the LBG at redshift z = 6.972 ± 0.002, with a rest-frame equivalent width EW rf = 13 Å. 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 ≃2%, and that of observing only one galaxy out of seven with S/N = 5 is ≃4%, but these can be as small as ∼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 Lyα 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 ∼ 6.5. We discuss the possibility that an increasingly neutral intergalactic medium is responsible for such quenching.