Investigating the Metal Line Systems at [CLC][ITAL]z[/ITAL][/CLC] = 1.9 toward J2233−606 in the Hubble Deep Field South

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Abstract
The combination of Space Telescope Imaging Spectrograph (STIS) and optical spectroscopy with STIS imaging of the field surrounding J2233-606 affords a unique opportunity to study the physical nature of the quasar absorption-line systems. We present an analysis of the ionization state, chemical abundances, and kinematic characteristics of two metal line systems at z = 1.92 and 1.94 toward J2233-606. We focus on these two systems because (1) the observations provide full coverage of the Lyman series, and hence an accurate determination of their H I column densities, and (2) they exhibit many metal line transitions that allow for a measurement of their ionization state and chemical abundances. Line profile fits of the Lyman series for the two systems indicate log N(H I) = 17.15 ± 0.02 cm-2 evenly distributed between two components for the z = 1.92 system and log N(H I) = 16.33 ± 0.04 cm-2 for the z = 1.94 system. By comparing observed ionic ratios of C and Si against calculations performed with the CLOUDY software package, we find the ionization state is high and well constrained in both systems. Applying ionization corrections to the measured ionic column densities, we determine that the systems exhibit significantly different metallicities: ≈1/50 solar and less than 1/200 solar for the two components of z = 1.92 and ≈40% solar at z = 1.94. The most significant uncertainties regarding these measurements are associated with the assumptions inherent to the CLOUDY simulations. The properties of the z = 1.92 system are consistent with a low-metallicity galaxy (e.g., a dwarf galaxy) as well as absorption by large-scale structure of the intergalactic medium. On the other hand, the high metallicity of the z = 1.94 system suggests significant star formation and is therefore assumed associated with a galactic system. Furthermore, we predict it is the more likely to be observed with the STIS imaging and follow-up observations. If a galaxy is identified, our results provide direct measurements on the properties of the interstellar medium for a z ≈ 2 galaxy.
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