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

Abstract

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 emitters within the continuum-selected dropout population. By quantifying how the 'Ly fraction', x Ly , varies with redshift, we seek to constrain changes in Ly transmission associated with reionization. In order to distinguish the effects of reionization from other factors which affect the Ly fraction [e.g. dust, interstellar medium (ISM) kinematics], we study the luminosity and redshift-dependence of the Ly fraction over 3 z 6, when the intergalactic medium (IGM) is known to be ionized. These results reveal that low-luminosity galaxies show strong Ly emission much more frequently (x Ly = 0.47 0.16 at M UV = -19) than luminous systems (x Ly = 0.08 0.02 at M UV = -21), and that at fixed luminosity, the prevalence of strong Lyman emission increases moderately with redshift over 3 < z < 6 (d x Ly /d z = 0.05 0.03). Based on the bluer mean UV slopes of the strong Ly emitting galaxies in our data set ( Ly - no Ly = -0.33 0.09 at M UV = -20.5) we argue that the Ly 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 7 galaxies, we find a tentative decrease in the Ly fraction by a factor of >1.9 with respect to the predicted z 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 fraction using optical and near-infrared multi-object spectrographs, thereby extending the study conducted in this paper to 7 z 8.