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 EWrf = 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 sime2%, and that of observing only one galaxy out of seven with S/N = 5 is sime4%, 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.
The Astrophysical Journal
- Pub Date:
- December 2010
- galaxies: distances and redshifts;
- galaxies: formation;
- galaxies: high-redshift;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- Submitted to ApJL: re-submitted version after modification responding to the referee's report