THE PHYSICAL CONDITIONS OF A LENSED STAR-FORMING GALAXY AT z=1.7

We report rest-frame optical Keck/NIRSPEC spectroscopy of the bright lensed galaxy RCSGA 032727-132609 at z = 1.7037. From precise measurements of the nebular lines, we infer a number of physical properties: redshift, extinction, star formation rate, ionization parameter, electron density, electron temperature, oxygen abundance, and N/O, Ne/O, and Ar/O abundance ratios. The limit on [O III] 4363 angstrom tightly constrains the oxygen abundance via the direct or T-e method, for the first time in an average-metallicity galaxy at z similar to 2. We compare this result to several standard bright-line O abundance diagnostics, thereby testing these empirically-calibrated diagnostics in situ. Finally, we explore the positions of lensed and unlensed galaxies in standard diagnostic diagrams, and the diversity of ionization conditions and mass-metallicity ratios at z = 2.