THE CHANGING Lyα OPTICAL DEPTH IN THE RANGE 6 < z < 9 FROM THE MOSFIRE SPECTROSCOPY OF Y-DROPOUTS

We present the MOSFIRE spectroscopy of 13 candidate z ~ 8 galaxies selected as Y-dropouts as part of the Brightest of Reionization Galaxies pure parallel survey. We detect no significant Ly? emission (our median 1? rest-frame equivalent width sensitivity is in the range 2-16 ?). Using the Bayesian framework derived in a previous paper, we perform a rigorous analysis of a statistical subsample of non-detections for 10 Y-dropouts, including data from the literature, to study the cosmic evolution of the Ly? emission of Lyman break galaxies. We find that Ly? emission is suppressed at z ~ 8 by at least a factor of three with respect to z ~ 6 continuing the downward trend found by previous studies of z-dropouts at z ~ 7. This finding suggests a dramatic evolution in the conditions of the intergalactic or circumgalactic media in just 300?Myr, consistent with the onset of reionization or changes in the physical conditions of the first generations of star-forming regions.

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