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

Abstract

We present the Magellan /FIRE detection of highly ionized C iv λ 1550 and O iii ] λ 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 Ly α . No corresponding emission is detected at the expected location of He ii λ 1640. The upper limit on He ii , paired with detection of O iii ] and C iv , constrains possible ionization scenarios. Production of C iv and O iii ] 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, He ii 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 He ii 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 population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.