Measuring chemical abundances in AGN from infrared nebular lines: HII-CHI-Mistry-IR for AGN

Future and on-going infrared and radio observatories such as JWST, METIS or ALMA will increase the amount of rest-frame IR spectroscopic data for galaxies by several orders of magnitude. While studies of the chemical composition of the ISM based on optical observations have been widely spread over decades for SFG and, more recently, for AGN, similar studies need to be perform using IR data which are less affected by temperature and dust extinction, trace high ionic species and provide robust estimations of the chemical abundance ratio N/O. We present a new tool based on a bayesian-like methodology (HII-CHI-Mistry-IR) to estimate chemical abundances from IR emission lines in AGN. We use a sample of 58 AGN with IR spectroscopic data retrieved from the literature to probe the validity of our method. IR estimations of chemical abundances in our sample composed by 43 Seyferts, 8 ULIRGs, 4 LIRGs and 3 LINERs are compared with those derived from optical emission lines. HII-CHI-Mistry-IR takes advantage of photoionization models to match a set of input emission lines with the best model characterized by the chemical abundance ratios O/H and N/O and the ionization parameter U. Instead of matching single emission lines, the code calculates sensitive ratios to the above free parameters. We report solar and subsolar abundances for O/H in the nuclear region for our sample of AGN, whereas N/O clusters around solar values. Compared to their optical estimations, there is a systematic discrepancy between IR and optical estimations of chemical abundances, being the later higher than the former. This result, consistent with previous studies of the composition of the ISM from IR observations, is independent from the density or the incident radiation field to the ISM.