KECK SPECTROSCOPY OF FAINT 3 > z > 7 LYMAN BREAK GALAXIES: A HIGH FRACTION OF LINE EMITTERS AT REDSHIFT SIX

As Ly alpha photons are scattered by neutral hydrogen, a change with redshift in the Ly alpha 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 Ly alpha 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 Ly alpha 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 similar or equal to 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 similar or equal to 6 LBGs show strong (W-Ly alpha,W-0 > 25 angstrom) emission, an increase of 55% from a sample of similarly luminous z similar or equal to 4 galaxies. With a total sample of 74 z similar or equal to 6 LBGs, we determine the luminosity-dependent Ly alpha EW distribution. Assuming continuity in these trends to the new population of z similar or equal to 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 Ly alpha emission should be high.