A New Constraint on the Ly$\alpha$ Fraction of UV Very Bright Galaxies at Redshift 7

TL;DR

This study investigates the presence of Lyman-alpha emission in very bright galaxies at redshift 7, providing new upper limits on the fraction exhibiting strong emission, which informs understanding of reionization and galaxy evolution.

Contribution

The paper presents new spectroscopic observations of bright z~7 galaxies and establishes the first upper limit on the Lyman-alpha emission fraction in this luminosity and redshift range.

Findings

Only one possible Lyman-alpha detection at z=7.168.

Upper limit of < 0.23 for Lya emitters with EW_0 > 50 Angstrom.

Weak trend suggesting the decline of strong Lya emitters from z~6 to z~7.

Abstract

We study the extent to which very bright (-23.0 < MUV < -21.75) Lyman-break selected galaxies at redshifts z~7 display detectable Lya emission. To explore this issue, we have obtained follow-up optical spectroscopy of 9 z~7 galaxies from a parent sample of 24 z~7 galaxy candidates selected from the 1.65 sq.deg COSMOS-UltraVISTA and SXDS-UDS survey fields using the latest near-infrared public survey data, and new ultra-deep Subaru z'-band imaging (which we also present and describe in this paper). Our spectroscopy has yielded only one possible detection of Lya at z=7.168 with a rest-frame equivalent width EW_0 = 3.7 (+1.7/-1.1) Angstrom. The relative weakness of this line, combined with our failure to detect Lya emission from the other spectroscopic targets allows us to place a new upper limit on the prevalence of strong Lya emission at these redshifts. For conservative calculation and to facilitate comparison with previous studies at lower redshifts, we derive a 1-sigma upper limit on the fraction of UV bright galaxies at z~7 that display EW_0 > 50 Angstrom, which we estimate to be < 0.23. This result may indicate a weak trend where the fraction of strong Lya emitters ceases to rise, and possibly falls between z~6 and z~7. Our results also leave open the possibility that strong Lya may still be more prevalent in the brightest galaxies in the reionization era than their fainter counterparts. A larger spectroscopic sample of galaxies is required to derive a more reliable constraint on the neutral hydrogen fraction at z~7 based on the Lya fraction in the bright galaxies.