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We present the results of our ultra-deep Keck/DEIMOS spectroscopy ofz-dropout galaxies in the SDF and GOODS-N. For 3 out of 11 objects, we detectan emission line at ~ 1um with a signal-to-noise ratio of ~ 10. The lines showasymmetric profiles with high weighted skewness values, consistent with beingLya, yielding redshifts of z=7.213, 6.965, and 6.844. Specifically, we confirmthe z=7.213 object in two independent DEIMOS runs with different spectroscopicconfigurations. The z=6.965 object is a known Lya emitter, IOK-1, for which ourimproved spectrum at a higher resolution yields a robust skewness measurement.The three z-dropouts have Lya fluxes of 3 x 10^-17 erg s^-1 cm^-2 andrest-frame equivalent widths EW_0^Lya = 33-43A. Based on the largestspectroscopic sample of 43 z-dropouts that is the combination of our andprevious data, we find that the fraction of Lya-emitting galaxies (EW_0^Lya >25A) is low at z ~ 7; 17 +- 10% and 24 +- 12% for bright (Muv ~= -21) and faint(Muv ~= -19.5) galaxies, respectively. The fractions of Lya-emitting galaxiesdrop from z ~ 6 to 7 and the amplitude of the drop is larger for faint galaxiesthan for bright galaxies. These two pieces of evidence would indicate that theneutral hydrogen fraction of the IGM increases from z ~ 6 to 7, and that thereionization proceeds from high- to low-density environments, as suggested byan inside-out reionization model.

Spectroscopic Confirmation of Three z-Dropout Galaxies at z = 6.844 - 7.213: Demographics of Lyman-Alpha Emission in z ~ 7 Galaxies