Keck spectroscopy of faint 3 < z < 7 Lyman break galaxies - I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman α emission

We present the first results of a new Keck spectroscopic survey of UV faint Lyman break galaxies in the redshift range 3 < z < 7. Combined with earlier Keck and published European Southern Observatory (ESO) VLT data, our spectroscopic sample contains more than 600 dropouts offering new insight into the nature of sub-L* sources typical of those likely to dominate the cosmic reionization process. In this first paper, in a series discussing these observations, we characterize the fraction of strong Ly alpha emitters within the continuum-selected dropout population. By quantifying how the 'Ly alpha fraction', x(Ly alpha), varies with redshift, we seek to constrain changes in Ly alpha transmission associated with reionization. In order to distinguish the effects of reionization from other factors which affect the Ly alpha fraction [e.g. dust, interstellar medium (ISM) kinematics], we study the luminosity and redshift-dependence of the Ly alpha fraction over 3 less than or similar to z less than or similar to 6, when the intergalactic medium (IGM) is known to be ionized. These results reveal that low-luminosity galaxies show strong Ly alpha emission much more frequently (x(Ly alpha) = 0.47 +/- 0.16 at M-UV = -19) than luminous systems (x(Ly alpha) = 0.08 +/- 0.02 at M-UV = -21), and that at fixed luminosity, the prevalence of strong Lyman alpha emission increases moderately with redshift over 3 < z < 6 (d x(Ly alpha)/d z = 0.05 +/- 0.03). Based on the bluer mean UV slopes of the strong Ly alpha emitting galaxies in our data set (<>(Ly alpha) - <>(no Ly alpha) = -0.33 +/- 0.09 at M-UV = -20.5) we argue that the Ly alpha fraction trends are governed by redshift and luminosity-dependent variations in the dust obscuration, with likely additional contributions from trends in the kinematics and covering fraction of neutral hydrogen. Using the limited infrared spectroscopy of candidate z similar or equal to 7 galaxies, we find a tentative decrease in the Ly alpha fraction by a factor of > 1.9 with respect to the predicted z similar or equal to 7 value, a result which, if confirmed with future surveys, would suggest an increase in the neutral fraction by this epoch. Given the abundant supply of z and Y drops now available from deep Hubble WFC3/IR surveys, we show it will soon be possible to significantly improve estimates of the Ly alpha fraction using optical and near-infrared multi-object spectrographs, thereby extending the study conducted in this paper to 7 less than or similar to z less than or similar to 8.