The changing Lya optical depth in the range 6<z<9 from MOSFIRE spectroscopy of Y-dropouts

TL;DR

This study investigates the evolution of Lyman-alpha emission in high-redshift galaxies between redshifts 6 and 9, revealing a significant suppression at z~8 indicative of cosmic reionization processes.

Contribution

It provides the first statistical analysis of Lyman-alpha emission suppression at z~8 using MOSFIRE spectroscopy and Bayesian methods, extending previous findings at lower redshifts.

Findings

Lyman-alpha emission is suppressed by at least a factor of three at z~8 compared to z~6.

The suppression trend continues from z~7 to z~8, indicating rapid evolution.

Results are consistent with the onset of reionization or changes in early galaxy environments.

Abstract

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.