Near-infrared Spectroscopy of Galaxies During Reionization: Measuring C III] in a Galaxy at z = 7.5

We present Keck/MOSFIRE H-band spectroscopy targeting C III] λ1907, 1909 in a z = 7.5056 galaxy previously identified via Lyα emission. We detect strong line emission at 1.621+/- 0.002 μ {{m}} with a line flux of (2.63 ± 0.52) × 10^-18 erg s^-1 cm^-2. We tentatively identify this line as [C III] λ1907, but we are unable to detect C III] λ1909 owing to sky emission at the expected location. This gives a galaxy systemic redshift, {z}_sys}=7.5032+/- 0.0003, with a velocity offset to Lyα of {{Δ }}{v}_Lyα } = 88 ± 27 km s^-1. The ratio of combined C III]/Lyα is 0.30-0.45, one of the highest values measured for any z > 2 galaxy. We do not detect Si III] λλ1883, 1892, and place an upper limit on Si III]/C III] < 0.35 (2σ). Comparing our results to photoionization models, the C III] equivalent width (W _{C III]} = 16.23 ± 2.32 Å), low Si III]/C III] ratio, and high implied [O III] equivalent width (from the Spitzer/IRAC [3.6]-[4.5] ≃ 0.8 mag color) require subsolar metallicities (Z ≃ 0.1-0.2 Z _⊙) and a high ionization parameter, log U ≳ -1.5. These results favor models that produce higher ionization, such as the BPASS models for the photospheres of high-mass stars, and that include both binary stellar populations and/or an IMF that extends to 300 M _⊙. The combined C III] equivalent width and [3.6]-[4.5] color are more consistent with ionization from young stars than active galactic nuclei (AGNs); however, we cannot rule out ionization from a combination of an AGN and young stars. We make predictions for James Webb Space Telescope spectroscopy using these different models, which will ultimately test the nature of the ionizing radiation in this source.