2018ApJ...864..103J

The distribution of Lyα emission is a presently accessible method for studying the state of the intergalactic medium (IGM) into the reionization era. We carried out deep spectroscopic observations in order to search for Lyα emission from galaxies with photometric redshifts z = 5.5-8.3 selected from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. Utilizing data from the Keck/DEIMOS spectrograph, we explore a wavelength coverage of Lyα emission at z ∼ 5-7 with 4 nights of spectroscopic observations for 118 galaxies, detecting five emission lines with ∼5σ significance: three in the GOODS-N and two in the GOODS-S fields. We constrain the equivalent-width (EW) distribution of Lyα emission by comparing the number of detected objects with the expected number constructed from detailed simulations of mock emission lines that account for the observational conditions (e.g., exposure time, wavelength coverage, and sky emission) and galaxy photometric redshift probability distribution functions. The Lyα EW distribution is well described by an exponential form, dN/dEW ∝ exp(-EW/W₀), characterized by the e-folding scale (W₀) of ∼60-100 Å at 0.3 < z < 6. By contrast, our measure of the Lyα EW distribution at 6.0 < z < 7.0 rejects a Lyα EW distribution with W₀ > 36.4 Å (125.3 Å) at 1σ (2σ) significance. This provides additional evidence that the EW distribution of Lyα declines at z > 6, suggesting an increasing fraction of neutral hydrogen in the IGM at that epoch.