Investigating the Metal Line Systems at Z=1.9 toward J2233-606 in the   HDF-S
  The combination of STIS and optical spectroscopy with STIS imaging of the field surrounding J2233-606 afford 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 z=1.94 toward J2233-606. We focus on these two systems because (i) the observations provide full coverage of the Lyman series, hence an accurate determination of their HI column densities and (ii) the metal-line transitions allow a measurement of their ionization state.   Line-profile fits of the Lyman series for the two systems indicate log N(HI) = 17.15 \pm 0.02 cm^-2 evenly distributed between two components for the z=1.92 system and log N(HI) = 16.33 \pm 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 < 1/200 solar for the two components of z=1.92 and ~40% solar at z=1.94. 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 IGM. On the other hand, the high metallicity of the z=1.94 system suggests significant star formation and is therefore most likely associated with a galactic system. Furthermore, we predict it is the most 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 ISM for a z~2 galaxy. 
Jason X. Prochaska (OCIW) and Scott M. Burles (U Chicago)