RAPID DECLINE OF Ly EMISSION TOWARD THE REIONIZATION ERA
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2014 The American Astronomical Society. All rights reserved. The observed deficit of strongly Ly emitting galaxies at z > 6.5 is attributed to increasing neutral hydrogen in the intergalactic medium (IGM) and/or to the evolving galaxy properties. To investigate this we have performed very deep near-IR spectroscopy of z 7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Ly fraction at z 8 using two methods. First, we derived N Ly/Ntotdirectly, using extensive simulations to correct for incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al.) that compares the z > 7 galaxy spectra to models of the Ly equivalent width (W Ly) distribution at z 6. We explored two simple evolutionary scenarios: pure number evolution where Ly is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines) and uniform dimming evolution where Ly is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity). The Bayesian formalism places stronger constraints compared with the direct method. Combining our data with that in the literature, we find that at z 8 the Ly fraction has dropped by a factor of >3 (84% confidence interval) using both the dimming and number evolution scenarios, compared to the z 6 values. Furthermore, we find a tentative positive Bayesian evidence favoring the number evolution scenario over dimming evolution, extending trends observed at z 7 to higher redshift. A comparison of our results with theoretical models implies the IGM volume averaged neutral hydrogen fraction 0.3, suggesting that we are likely witnessing reionization in progress at z 8.
