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] lambda 1907, 1909 in a z = 7.5056 galaxy previously identified via Ly alpha emission. We detect strong line emission at 1.621 +/- 0.002 mu m with a line flux of (2.63 +/- 0.52) x 10(-18) erg s(-1) cm(-2). We tentatively identify this line as [C III] lambda 1907, but we are unable to detect C III] lambda 1909 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 Ly alpha of Delta nu(Ly alpha) = 88 +/- 27 km s(-1). The ratio of combined C III]/Ly alpha is 0.30-0.45, one of the highest values measured for any z > 2 galaxy. We do not detect Si III] lambda lambda 1883, 1892, and place an upper limit on Si III]/C III] 0.35 (2 sigma). Comparing our results to photoionization models, the C III] equivalent width (W-C III] = 16.23 +/- 2.32 angstrom), low Si III]/C III] ratio, and high implied [O III] equivalent width (from the Spitzer/IRAC [3.6]-[4.5] similar or equal to 0.8 mag color) require subsolar metallicities (Z similar or equal to 0.1-0.2 Z(circle dot)) and a high ionization parameter, log U greater than or similar to -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-circle dot. 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.