Context :

Aims : We aim to determine the redshift of GDS_1408 , the most solid z \sim 7 galaxy candidate lying in the Hubble Ultra Deep Field .

Methods : We have used all the VLT spectra of GDS_1408 collected by us and two other groups with FORS2 at VLT in the last five years , for a total integration time of 52hr .
The combined spectrum is the deepest ever obtained of a galaxy in the Reionization epoch .

Results : We do not detect any emission line or continuum over the whole wavelength range , up to 10100Å .
Based on an accurate set of simulations , we are able to put a stringent upper limit of f ( Ly \alpha ) < 3 \times 10 ^ { -18 } ~ { } erg / s / cm ^ { 2 } at 3-9 sigma in the explored wavelength range , corresponding to a rest-frame equivalent width EW < 9 Å .
Combining this limit with the SED modelling we refine the redshift to be z = 6.82 \pm 0.1 ( 1-sigma ) .
The same SED fitting indicates that GDS_1408 is relatively extinct ( A _ { 1600 } \simeq 1 ) with a dust corrected star formation rate of \simeq 20 M _ { \odot } yr ^ { -1 } .
The comparison between the un-attenuated equivalent width predicted by the case-B recombination theory and the observed upper limit , provides a limit on the effective Ly \alpha escape fraction of f _ { esc } ^ { eff } ( Ly \alpha ) < 8 % .
Even though we can not rule out a major contribution of the inter/circum galactic medium in damping the line , a plausible interpretation is that G2_1408 is moderately evolved and contains sufficient gas and dust to attenuate the Ly \alpha emission , before it reaches the intergalactic medium .

Conclusions : The redshift confirmation of even the best z \simeq 7 candidates is very hard to achieve ( unless the Ly \alpha or unusually strong rest-UV nebular emission lines are present ) with the current generation of 8-10m class telescopes .
We show that both JWST and E-ELT will be necessary to make decisive progresses .
Currently , the increased redshift accuracy obtained with this kind of analysis makes ALMA an interesting option for the redshift confirmation .