2014A&A...569A..78V

We aim to determine the redshift of GDS_1408, the most solid z∼7 galaxy candidate lying in the Hubble Ultra Deep Field. We have used all the VLT spectra for GDS_1408 that has been collected by two other groups with FORS2 at VLT and by us in the last five years for a total integration time of 52h. The combined spectrum is the deepest ever obtained of a galaxy in the reionization epoch. We do not detect any emission line or continuum up to 10100Å. Based on an accurate set of simulations, we are able to put a stringent upper limit of f(Lyα)<3x10^–18erg/s/cm² at 3-9 sigma in the explored wavelength range, which corresponds to a rest-frame equivalent width EW<9Å. Combining this limit with the spectral energy distribution (SED) modeling, we refine the redshift to be z=6.82 ±0.1 (1-sigma). The same SED fitting indicates that GDS_1408 is relatively extinct (A₁₆₀₀≃1) with a dust corrected star-formation rate of ≃20M_☉/yr. 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 Lyα escape fraction of f_esc^eff(Lyα)<8%. Even though we cannot 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 Lyα emission before it reaches the intergalactic medium. The redshift confirmation of even the best z≃7 candidates is very hard to achieve (unless the Lyα or unusually strong rest-UV nebular emission lines are present) with the current generation of 8-10 m class telescopes. We show that both JWST and E-ELT are 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.