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 SL2SJ021737-051329, a lens system consisting of a bright arc at z=1.84435, magnified ~17x by a massive galaxy at z=0.65. SL2SJ0217 is a low-mass (M <10^9 M*), low-metallicity (Z~1/20 Z*) 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 CIV 1548,1550, HeII 1640, OIII] 1661,1666, SiIII] 1883,1892, and CIII] 1907,1909. (2) Double-peaked LyA 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 sightline 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 an additional ionization source is needed. We rule out large contributions from AGN 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, SL2S0217 serves as a template for the extreme conditions that are important for reionization and thought to be more common in the early Universe.Comment: 28 pages, 16 figures, 8 tables, re-submitted to ApJ, comments welcom

Aperture and Resolution Effects on Ultraviolet Star-Forming Properties: Insights from Local Galaxies and Implications for High-Redshift Observations

We present an analysis of the effects of spectral resolution and aperture scales on derived galaxy properties using far-ultraviolet (FUV) spectra of local star-forming galaxies from the International Ultraviolet Explorer (R~250, FOV~10"x20") and Cosmic Origins Spectrograph on the Hubble Space Telescope (R~15,000, FOV~2.5"). Using these spectra, we measured FUV luminosities, spectral slopes, dust attenuation, and equivalent widths. We find that galaxies with one dominant stellar cluster have FUV properties that are independent of aperture size, while galaxies with multiple bright clusters are sensitive to the total light fraction captured by the aperture. Additionally, we find significant correlations between the strength of stellar and interstellar absorption-lines and metallicity, indicating metallicity-dependent line-driven stellar winds and interstellar macroscopic gas flows shape the stellar and interstellar spectral lines, respectively. The observed line-strength versus metallicity relation of stellar-wind lines agrees with the prediction of population synthesis models for young starbursts. In particular, measurements of the strong stellar CIV 1548,1550 line provide an opportunity to determine stellar abundances as a complement to gas-phase abundances. We provide a relation between the equivalent width of the CIV line and the oxygen abundance of the galaxy. We discuss this relation in terms of the stellar-wind properties of massive stars. As the driving lines in stellar winds are mostly ionized iron species, the CIV line may eventually offer a method to probe alpha-element-to-iron ratios in star-forming galaxies once consistent models with non-solar abundance ratios are available. These results have important implications for the galaxy-scale, low-resolution observations of high-redshift galaxies from JWST (R~100-3,500).Comment: This paper has 31 pages total, 11 figures, and a figureset. Accepted for publication in Ap