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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-massgalaxy RXC J2248.7-4431-ID3, which has previously-known Lyman-alpha. Nocorresponding emission is detected at the expected location of HeII 1640. Theupper limit on HeII paired with detection of OIII] and CIV constrains possibleionization scenarios. Production of CIV and OIII] requires ionizing photons of2.5-3.5 Ryd, but once in that state their multiplet emission is powered bycollisional excitation at lower energies (~0.5 Ryd). As a pure recombinationline, HeII emission is powered by 4 Ryd ionizing photons. The data thereforerequire a spectrum with significant power at 3.5 Ryd but a rapid drop toward4.0 Ryd. This hard spectrum with a steep drop is characteristic oflow-metallicity stellar populations, and less consistent with soft AGNexcitation, which features more 4 Ryd photons and hence higher HeII flux. Theconclusions based on ratios of metal line detections to Helium non-detectionare strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growinghandful of reionization-era galaxies with UV emission line ratios distinct fromthe general z=2-3 population, in a way that suggests hard ionizing spectra thatdo not necessarily originate in AGN.

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