The changing Lya optical depth in the range 6<z<9 from MOSFIRE
  spectroscopy of Y-dropouts
  We present MOSFIRE spectroscopy of 13 candidate z~8 galaxies selected as
Y-dropouts as part of the BoRG pure parallel survey. We detect no significant
lya emission (our median 1-sigma rest frame equivalent width sensitivity is in
the range 2-16 AA). Using the Bayesian framework derived in a previous paper,
we perform a rigorous analysis of a statistical subsample of non-detections for
ten Y-dropouts, including data from the literature, to study the cosmic
evolution of the lya emission of Lyman Break Galaxies. We find that lya
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 Myrs, consistent with the
onset of reionization or changes in the physical conditions of the first
generations of starforming regions.
