Improving the estimation of iron abundance in ionized nebulae

Ionized nebulae, such as H II regions and star-forming galaxies (SFGs), are key environments for studying the chemical composition of the interstellar medium at different spatial and temporal scales. Iron (Fe), a crucial element in these studies, is particularly difficult to measure accurately due to its tendency to condense onto dust grains and the weakness of its optical emission lines. The main goal of this work is to improve the estimation of the total Fe abundance in ionized nebulae by developing a new ionization correction factor (ICF) for Fe2+, based on updated photoionization models. A filtered grid of photoionization models from the 3MdB/BOND project is used, selecting those that represent typical conditions in H II regions and SFGs. From these models, a new ICF(Fe2+) is constructed as a function of the ionization degree (O2+/O), using a binning method with two overlapping intervals. This ICF(Fe2+) is applied to a subset of 430 high-quality optical spectra from the DESIRED-E observational dataset, which includes reliable detections of the [Fe III] λ4658 emission line. This new function shows better consistency with physical models and provides lower Fe/H estimates than those obtained using classical ICFs by Izotov et al. (2006) and Rodríguez & Rubin (2005), especially in intermediate and high ionization regimes. Moreover, the abundances derived with the new ICF(Fe2+) shows better agreement with those obtained from the direct sum of ionic species in a subset of well-observed regions. The new ICF(Fe2+) represents a significant improvement over previous formulations, thanks to the use of more realistic models and updated atomic data.