HII-CHI-Mistry-UV for AGN. Abundances

We evaluate different ultraviolet emission line ratios and how different assumptions about the models, including the presence of dust grains, the shape of the incident spectral energy distribution, or the thickness of the gas envelope around the central source, may affect the final estimates as a function of the set of emission lines used.

We compare our results with other published recipes for deriving abundances using the same emission lines and show that deriving the carbon-to-oxygen abundance ratio using CIII] {lambda}1909{AA} and OIII] {lambda}1665{AA} emission lines is a robust indicator of the metal content in AGN that is nearly independent of model assumptions, similar to the case of star-forming regions. Moreover, we show that a prior determination of C/O allows for a much more precise determination of the total oxygen abundance using carbon UV lines, as op- posed to assuming an arbitrary relationship between O/H and C/O, which can lead to non-negligible discrepancies.