2002ApJ...571..693P

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: Ar⁰/S⁺, Fe⁺⁺/Fe⁺, N⁺/N⁰, 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⁺/N⁰, Al⁺⁺/Al⁺, and Ar⁰/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 Ar⁰. 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 (N⁰, Ar⁰)/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 Ar⁰/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.