We present ultradeep optical spectroscopy obtained with FORS2 on 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 \leq 7 ) , located at \lambda = 9691.5 \pm 0.5 Å and with flux 3.4 \times 10 ^ { -18 } erg cm ^ { -2 } s ^ { -1 } .
If identified as Ly \alpha , it places the LBG at redshift z = 6.972 \pm 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 \simeq 2 % , and that of observing only one galaxy out of seven with S / N = 5 is \simeq 4 % , but these can be as small as \sim 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 Ly \alpha 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 \sim 6.5 .
We discuss the possibility that an increasingly neutral intergalactic medium is responsible for such quenching .