A 52 hours VLT/FORS2 spectrum of a bright z~7 HUDF galaxy: no Ly-alpha
  emission
  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 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. We
do not detect any emission line or continuum over the whole wavelength range,
up to 10100A. Based on an accurate set of simulations, we are able to put a
stringent upper limit of f(Lya)<3x10^(-18) erg/s/cm2 at 3-9 sigma in the
explored wavelength range, corresponding to a rest-frame equivalent width
EW<9A. Combining this limit with the SED modelling 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 ~ 20
Msol/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 Lya escape fraction of f_(esc)^(eff)(Lya)<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 GDS-1408 is moderately
evolved and contains sufficient gas and dust to attenuate the Lya emission,
before it reaches the intergalactic medium. The redshift confirmation of even
the best z~7 candidates is very hard to achieve (unless the Lya 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.
