Chemical Abundance Calibrations for the Narrow-Line Region of Active Galaxies

We investigate two chemical abundance calibrations for the narrow-line-region(NLR) of active galaxies in terms of three easily observable opticalemission-line ratios, namely, [OIII]$\lambda\lambda$4959,5007/H$\beta$,[NII]$\lambda\lambda$6548,84/H$\alpha$ and[OII]$\lambda3727/$[OIII]$\lambda\lambda$4959,5007. The calibrations areobtained from a grid of models on the assumption that the main processresponsible for the production of these lines is photoionization by a``typical'' active galactic nucleus continuum. The chemical elements vary theirabundance together with oxygen, except nitrogen, which is assumed to be aproduct of secondary nucleosynthesis. The calibrations are calculated for therange $8.4 \le 12+log(O/H) \le 9.4$, and tested using NLR data for a sample ofSeyfert's and LINER's having HII regions in the vicinity of the nucleus. Thegaseous abundances of these HII regions have been determined in previous works,and the NLR abundances are obtained on the assumption that they can beextrapolated from those of the HII regions. The calibrations work very well forthe Seyfert's, giving abundance values which agree with those obtained from theHII regions, and can thus be used for quick estimates of the chemicalabundances of the NLR's. For the LINER's, the calibrations give lower valuesthan those derived from the corresponding HII regions, suggesting that theassumptions of the models do not apply for them, and that there are differentphysical processes at work in the NLR of the LINER's.Comment: 14 pages, 4 figures, accepted for publication in The Astronomical Journa