Spectroscopic confirmation of three z-dropout galaxies at z = 6.844-7.213: Demographics of Lyα emission in z ∼ 7 galaxies

Abstract

We present the results of our ultra-deep Keck/DEIMOS spectroscopy of z-dropout galaxies in the Subaru Deep Field and Great Observatories Origins Deep Survey's northern field. For 3 out of 11 objects, we detect an emission line at ∼ 1 μm with a signal-to-noise ratio of ∼10. The lines show asymmetric profiles with high weighted skewness values, consistent with being Lyα, yielding redshifts of z = 7.213, 6.965, and 6.844. Specifically, we confirm the z = 7.213 object in two independent DEIMOS runs with different spectroscopic configurations. The z = 6.965 object is a known Lyα emitter, IOK-1, for which our improved spectrum at a higher resolution yields a robust skewness measurement. The three z-dropouts have Lyα fluxes of 3 × 10-17ergs-1cm-2 and rest-frame equivalent widths EWLyα0 = 33-43 . Based on the largest spectroscopic sample of 43 z-dropouts, which is the combination of our and previous data, we find that the fraction of Lyα-emitting galaxies (EW Lyα0 > 25 Å ) is low at z ∼ 7; 17% 10% and 24% ± 12% for bright (M UV ≃ -21) and faint (M UV ≃ -19.5) galaxies, respectively. The fractions of Lyα-emitting galaxies drop from z ∼ 6 to 7 and the amplitude of the drop is larger for faint galaxies than for bright galaxies. These two pieces of evidence would indicate that the neutral hydrogen fraction of the intergalactic medium increases from z ∼ 6 to 7 and that the reionization proceeds from high- to low-density environments, as suggested by an inside-out reionization model. © 2012. The American Astronomical Society. All rights reserved.