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We present Keck/MOSFIRE observations of UV metal lines in four brightgravitationally-lensed z~6-8 galaxies behind the cluster Abell 1703. Thespectrum of A1703-zd6, a highly-magnified star forming galaxy with aLyman-alpha redshift of z=7.045, reveals a confident detection of the nebularCIV emission line (unresolved with FWHM < 125 km/s). UV metal lines are notdetected in the three other galaxies. At z~2-3, nebular CIV emission isobserved in just 1% of UV-selected galaxies. The presence of strong CIVemission in one of the small sample of galaxies targeted in this paper mayindicate hard ionizing spectra are more common at z~7. The total estimatedequivalent width of the CIV doublet (38 A) and CIV/Lyman-alpha flux ratio (0.3)are comparable to measurements of narrow-lined AGNs. Photoionization modelsshow that the nebular CIV line can also be reproduced by a young stellarpopulation, with very hot metal poor stars dominating the photon fluxresponsible for triply ionizing carbon. Regardless of the origin of the CIV, weshow that the ionizing spectrum of A1703-zd6 is different from that of typicalgalaxies at z~2, producing more H ionizing photons per unit 1500A luminosityand a larger flux density at 30-50 eV. If such extreme radiation fields aretypical in UV-selected systems at z>7, it would indicate that reionization-eragalaxies are more efficient ionizing agents than previously thought.Alternatively, we suggest that the small sample of Lyman-alpha emitters at z>7may trace a rare population with intense radiation fields capable of ionizingtheir surrounding hydrogen distribution. Additional constraints on highionization emission lines in galaxies with and without Lyman-alpha detectionswill help clarify whether hard ionizing spectra are common in the reionizationera.

Spectroscopic detection of CIV in a galaxy at z=7.045: Implications for the ionizing spectra of reionization-era galaxies