SPECTROSCOPIC CONFIRMATION OF THREE z-DROPOUT GALAXIES AT z=6.844-7.213: DEMOGRAPHICS OF Lyα EMISSION IN z ∼ 7 GALAXIES

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 similar to 1 mu m with a signal-to-noise ratio of similar to 10. The lines show asymmetric profiles with high weighted skewness values, consistent with being Ly alpha, 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 Ly alpha emitter, IOK-1, for which our improved spectrum at a higher resolution yields a robust skewness measurement. The three z-dropouts have Ly alpha fluxes of 3x10(-17) erg s(-1) cm(-2) and rest-frame equivalent widths EW0Ly alpha = 33-43 angstrom. 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 Ly alpha-emitting galaxies (EW0Ly alpha > 25 angstrom) is low at z similar to 7; 17% +/- 10% and 24% +/- 12% for bright (M-UV similar or equal to -21) and faint (M-UV similar or equal to -19.5) galaxies, respectively. The fractions of Ly alpha-emitting galaxies drop from z similar to 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 similar to 6 to 7 and that the reionization proceeds from high-to low-density environments, as suggested by an inside-out reionization model.