A Window on the Earliest Star Formation: Extreme Photoionization Conditions of a High-ionization, Low-metallicity Lensed Galaxy at z ∼ 2

We report new observations of SL2S J021737-051329, a lens system consisting of a bright arc at z = 1.84435, magnified similar to 17 x by a massive galaxy at z = 0.65. SL2S0217 is a low-mass (M < 10(9) M-circle dot, low-metallicity (Z similar to 1/20 Z(circle dot)) galaxy, with extreme star-forming conditions that produce strong nebular UV emission lines in the absence of any apparent outflows. Here we present several notable features from rest-frame UV Keck/LRIS spectroscopy: (1) Very strong narrow emission lines are measured for C IV lambda lambda 1548, 1550, He II lambda 1640, O III lambda lambda 1661, 1666, Si III] lambda lambda 1883, 1892, and C lambda lambda 1907, 1909. (2) Double-peaked Ly alpha emission is observed with a dominant blue peak and centered near the systemic velocity. (3) The low- and high-ionization absorption features indicate very little or no outflowing gas along the sight line to the lensed galaxy. The relative emission-line strengths can be reproduced with a very high ionization, low-metallicity starburst with binaries, with the exception of He II, which indicates that an additional ionization source is needed. We rule out large contributions from active galactic nuclei and shocks to the photoionization budget, suggesting that the emission features requiring the hardest radiation field likely result from extreme stellar populations that are beyond the capabilities of current models. Therefore, 5L250217 serves as a template for the extreme conditions that are important for reionization and thought to be more common in the early universe.