Evidence for a Hard Ionizing Spectrum from a z=6.11 Stellar Population

TL;DR

This study detects highly-ionized metal lines in a z=6.11 galaxy, indicating a hard stellar ionizing spectrum with a steep drop at 4 Ryd, suggesting low-metallicity stellar populations rather than AGN as the source.

Contribution

It provides observational evidence of a steeply declining ionizing spectrum in a high-redshift galaxy, challenging the assumption that such spectra are primarily AGN-driven.

Findings

Detection of CIV and OIII] lines at z=6.11.

Absence of HeII emission constrains the ionizing spectrum.

Supports low-metallicity stellar populations as the ionizing source.

Abstract

We present the Magellan/FIRE detection of highly-ionized CIV 1550 and OIII] 1666 in a deep infrared spectrum of the z=6.11 gravitationally lensed low-mass galaxy RXC J2248.7-4431-ID3, which has previously-known Lyman-alpha. No corresponding emission is detected at the expected location of HeII 1640. The upper limit on HeII paired with detection of OIII] and CIV constrains possible ionization scenarios. Production of CIV and OIII] requires ionizing photons of 2.5-3.5 Ryd, but once in that state their multiplet emission is powered by collisional excitation at lower energies (~0.5 Ryd). As a pure recombination line, HeII emission is powered by 4 Ryd ionizing photons. The data therefore require a spectrum with significant power at 3.5 Ryd but a rapid drop toward 4.0 Ryd. This hard spectrum with a steep drop is characteristic of low-metallicity stellar populations, and less consistent with soft AGN excitation, which features more 4 Ryd photons and hence higher HeII flux. The conclusions based on ratios of metal line detections to Helium non-detection are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing handful of reionization-era galaxies with UV emission line ratios distinct from the general z=2-3 population, in a way that suggests hard ionizing spectra that do not necessarily originate in AGN.