A 52 hours VLT/FORS2 spectrum of a bright z ~ 7 HUDF galaxy: no Ly-α emission^⋆

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: eros.vanzella@oabo.inaf.it
² INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
³ Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA
⁴ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
⁵ Dipartimento di Fisica e Astronomia, Universita’ degli Studi di Bologna, Viale Berti-Pichat 6/2, 40127 Bologna, Italy

Received: 27 May 2014
Accepted: 11 July 2014

Abstract

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

Methods. 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 52 h. 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 up to 10 100 Å. Based on an accurate set of simulations, we are able to put a stringent upper limit of f(Lyα) <3 × 10^-18 erg/s/cm² at 3–9 sigma in the explored wavelength range, which corresponds to a rest-frame equivalent width EW< 9 Å. 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 ≃20 M_⊙ 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 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.

Conclusions. 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.