Spectroscopic Confirmation of z ~ 7 Lyman Break Galaxies: Probing the Earliest Galaxies and the Epoch of Reionization
Abstract
We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO Very Large Telescope (VLT) for the confirmation of z ~= 7 "z-band dropout" candidates selected from our VLT/Hawk-I imaging survey over three independent fields. In particular, we report on two newly discovered galaxies at redshift ~6.7 in the New Technology Telescope Deep Field. Both galaxies show an Lyα emission line with rest-frame equivalent widths (EWs) of the order of 15-20 Å and luminosities of (2-4) × 1042 erg s-1. We also present the results of ultra-deep observations of a sample of i-dropout galaxies, from which we set a solid upper limit on the fraction of interlopers. Out of the 20 z-dropouts observed we confirm 5 galaxies at 6.6 < z < 7.1. This is systematically below the expectations drawn on the basis of lower redshift observations: in particular, there is a significant lack of objects with intermediate Lyα EWs (between 20 and 55 Å). We conclude that the observed trend for the rising fraction of Lyα emission in Lyman break galaxies from z ~ 3 to z ~ 6 is most probably reversed from z ~ 6 to z ~ 7. Explaining the observed rapid change in the Lyα emitter fraction among the dropout population with reionization requires a fast evolution of the neutral fraction of hydrogen in the universe. Assuming that the universe is completely ionized at z = 6 and adopting a set of semi-analytical models, we find that our data require a change of the neutral hydrogen fraction of the order of Δ χ_{H I} ∼ 0.6 in a time Δz ~ 1, provided that the escape fraction does not increase dramatically over the same redshift interval.
We would like to dedicate this paper in memory of Alan Moorwood, who left us a few days before the paper was submitted. Alan was fundamental to the development of Hawk-I, which enabled this survey and many other important observing programs. He had clear foresight of the instrument's impact on the search for the highest redshift galaxies. More importantly, he always urged us to obtain spectroscopic confirmation of the candidates and was eagerly awaiting the results of this effort.- Publication:
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The Astrophysical Journal
- Pub Date:
- December 2011
- DOI:
- 10.1088/0004-637X/743/2/132
- arXiv:
- arXiv:1107.1376
- Bibcode:
- 2011ApJ...743..132P
- Keywords:
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- galaxies: distances and redshifts;
- galaxies: formation;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- Submitted to ApJ