The UCSD HIRES/Keck I damped Lyα abundance database.: III.: An empirical study of photoionization in the damped Lyα system toward GB 1759+7539

We investigate the ionization state of the damped Lyalpha 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 (≲ 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.