THE UCSD HIRES/KECK Ⅰ DAMPED Lyα ABUNDANCE DATABASE. Ⅲ. AN EMPIRICAL STUDY OF PHOTOIONIZATION IN THE DAMPED Lyα SYSTEM TOWARD GB 1759+7539

摘要

We investigate the ionization state of the damped Lyα system at z = 2.62 toward GB 1759+7539 through an analysis of ionic ratios sensitive to photoionization: Ar~0/S~+, Fe~(++)/Fe~+, N~+/N~0, and Al~(++)/Al~+. Approximately half of the metals arise in a mostly neutral velocity component with H Ⅰ/H > 0.9, based on Fe~(++)/Fe~+ < 0.013. In contrast, the remaining half exhibit Fe~(++)/Fe~+ ≈ 0.3, indicative of a partially ionized medium with H Ⅰ/H ≈ 0.5. These conclusions are supported by the observed N~+/N~0, Al~(++)/Al~+, and Ar~0/Si~+ ratios. We assess ionization corrections for the observed column densities through photoionization models derived from the CLOUDY software package. In the neutral gas, the ionization corrections are negligible, except for Ar~0. However, for the partially ionized gas, element abundance ratios differ from the ionic ratios by 0.1-0.3 dex for (Si~+, S~+, Ni~+, Al~+)/Fe~+ ratios and more for (N~0, Ar~0)/Fe~+. Independent of the shape of the photoionizing spectrum and assumptions of the number of ionization phases, these ionization corrections have minimal impact (approx< 0.1 dex) on the total metallicity inferred for this damped Lyα system. Measurements of the relative elemental abundances of the partially ionized gas, however, have a greater than ≈0.15 dex uncertainty, which hides the effects of nucleosynthesis and differential dust depletion. We caution the reader that this damped system is unusual for a number of reasons (e.g., a very low Ar~0/S~+ ratio), and we believe its ionization properties are special but not unique. Nevertheless, it clearly shows the value of examining photoionization diagnostics such as Fe~(++)/Fe~+ in a larger sample of damped systems.