The lack of intense Lyman~alpha in ultradeep spectra of z=7 candidates in GOODS-S: imprint of reionization?

Research paper by A. Fontana, E. Vanzella, L. Pentericci, M. Castellano, M. Giavalisco, A. Grazian, K. Boutsia, S. Cristiani, M. Dickinson, E. Giallongo, M. Maiolino, A. Moorwood, P. Santini

Indexed on: 13 Oct '10Published on: 13 Oct '10Published in: arXiv - Astrophysics - Cosmology and Nongalactic Astrophysics


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< 7), located at 9691.5 +/- 0.5A and with flux 3.4 x 10^(-18)erg/cm^2/s. If identified as Lyman alpha, it places the LBG at redshift z=6.972+/- 0.002, with a rest-frame equivalent width EW}=13A. 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 ~ 2%, and that of observing only one galaxy out of seven with S/N=5 is ~4%, but these can be as small as ~1E-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 Lyman 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~ 6.5. We discuss the possibility that an increasingly neutral intergalactic medium is responsible for such quenching.