A remarkably high fraction of strong Ly_alpha emitters amongst luminous redshift 6.0<z<6.5 Lyman break galaxies in the UKIDSS Ultra-Deep Survey

TL;DR

This study confirms a high fraction of luminous z~6.0-6.5 Lyman break galaxies exhibit strong Ly_alpha emission, indicating an increase in Ly_alpha emitter fraction among luminous galaxies near reionization.

Contribution

It provides the first spectroscopic confirmation of a high Ly_alpha emitter fraction among luminous z~6 galaxies, highlighting a potential luminosity-dependent evolution during reionization.

Findings

40%-50% of luminous galaxies show strong Ly_alpha emission

Spectroscopic confirmation rate of >= 71% for the sample

Ly_alpha escape fraction estimated at ~25%

Abstract

We present spectroscopic confirmation of ten highly luminous (L >= 2L*) Lyman alpha emitters in the redshift range 6.01<z<6.49 (nine galaxies and one AGN), initially drawn from a sample of fourteen z_phot >= 6 Lyman break galaxies (LBGs) selected from an area of 0.25 square degrees within the UKIDSS Ultra-deep Survey (UDS). Overall, our high rate of spectroscopic confirmation (>= 71%) and low rate of contamination provides a strong vindication of the photometric redshift analysis used to define the original sample. By considering star-formation rate estimates based on the Ly_alpha and UV continuum luminosity we conclude that our sample is consistent with a Ly_alpha escape fraction of ~25%. Moreover, after careful consideration of the potential uncertainties and biases, we find that 40%-50% of our sample of L >= 2L* galaxies at 6.0<z<6.5 display strong Ly_alpha emission (rest-frame equivalent width >= 25 Angs), a fraction which is a factor of ~2 higher than previously reported for L <= L* galaxies at z~6. Our results suggest that, as the epoch of reionization is approached, it is plausible that the Ly_alpha emitter fraction amongst luminous (L >=2 L*) LBGs shows a similarly sharp increase to that observed in their lower-luminosity (L <= L*) counterparts.