Spectroscopic Constraints on UV Metal Line Emission at z~6-9: The Nature of Lyman-alpha Emitting Galaxies in the Reionization-Era

TL;DR

This study investigates UV metal emission lines in high-redshift Lyman-alpha emitters, revealing that intense radiation fields, possibly from AGN or shocks, are common and may influence Lyman-alpha visibility during reionization.

Contribution

It provides the first detection of NV emission in a z~8.7 galaxy and shows that extreme radiation fields are prevalent among Lyman-alpha emitters in the reionization era.

Findings

Detection of NV emission in a z=8.683 galaxy suggests a hard ionizing spectrum.

Most high-redshift Lyman-alpha emitters lack UV metal lines, indicating diversity.

Extreme radiation fields are common among Lyman-alpha emitters at z>7.

Abstract

Recent studies have revealed intense UV metal emission lines in a modest sample of z>7 Lyman-alpha emitters, indicating a hard ionizing spectrum is present. If such high ionization features are shown to be common, it may indicate that extreme radiation fields play a role in regulating the visibility of Lyman-alpha in the reionization era. Here we present deep near-infrared spectra of seven galaxies with Lyman-alpha emission at 5.4<z<8.7 (including a newly-confirmed lensed galaxy at z=6.031) and three bright z>7 photometric targets. In nine sources we do not detect UV metal lines. However in the z=8.683 galaxy EGSY8p7, we detect a 4.6 sigma emission line in the narrow spectral window expected for NV 1243. The feature is unresolved (FWHM<90 km/s) and is likely nebular in origin. A deep H-band spectrum of EGSY8p7 reveals non-detections of CIV, He II, and OIII]. The presence of NV requires a substantial flux of photons above 77 eV, pointing to a hard ionizing spectrum powered by an AGN or fast radiative shocks. Regardless of its origin, the intense radiation field of EGSY8p7 may aid the transmission of Lyman-alpha through what is likely a partially neutral IGM. With this new detection, five of thirteen known Lyman-alpha emitters at z>7 have now been shown to have intense UV line emission, suggesting that extreme radiation fields are commonplace among the Lyman-alpha population. Future observations with JWST will eventually clarify the origin of these features and explain their role in the visibility of Lyman-alpha in the reionization era.