Keck Spectroscopy of Faint 3>z>7 Lyman Break Galaxies: A High Fraction of Line Emitters at Redshift Six

As Lyα photons are scattered by neutral hydrogen, a change with redshift in the Lyα equivalent width (EW) distribution of distant galaxies offers a promising probe of the degree of ionization in the intergalactic medium and hence when cosmic reionization ended. This simple test is complicated by the fact that Lyα emission can also be affected by variations in the kinematics and dust content of the host galaxies. In the first paper in this series, we demonstrated both a luminosity- and redshift-dependent trend in the fraction of Lyα emitters seen within color-selected "Lyman break" galaxies (LBGs) over the range 3 < z < 6; lower luminosity galaxies and those at higher redshift show an increased likelihood of strong emission. Here, we present the results from 12.5 hr exposures with the Keck DEIMOS spectrograph focused primarily on LBGs at z ~= 6 which enable us to confirm the redshift dependence of line emission more robustly and to higher redshift than was hitherto possible. We find that 54% ± 11% of faint z ~= 6 LBGs show strong (W _Lyα,0>25 Å) emission, an increase of 55% from a sample of similarly luminous z ~= 4 galaxies. With a total sample of 74 z ~= 6 LBGs, we determine the luminosity-dependent Lyα EW distribution. Assuming continuity in these trends to the new population of z ~= 7 sources located with the Hubble WFC3/IR camera, we predict that unless the neutral fraction rises in the intervening 200 Myr, the success rate for spectroscopic confirmation using Lyα emission should be high.