Chemical Abundance Calibrations for the Narrow-Line Region of Active Galaxies
Abstract
We investigate two chemical abundance calibrations for the narrow-line region (NLR) of active galaxies in terms of three easily observable optical emission-line ratios, namely, [O iii] lambdalambda4959, 5007/Hβ [N ii] lambdalambda6548, 6584/Hα and [O ii] lambda3727/[O iii] lambdalambda4959, 5007. The calibrations are obtained from a grid of models on the assumption that the main process responsible for the production of these lines is photoionization by a ``typical'' active galactic nucleus continuum. The elemental abundances vary with oxygen, except nitrogen, which is assumed to be a product of secondary nucleosynthesis. The calibrations are calculated for the range 8.4 <= 12 + log (O/H) <= 9.4 and tested using NLR data for a sample of Seyfert galaxies and LINERs that have H ii regions in the vicinity of the nucleus. The gaseous abundances of these H ii regions have been determined in previous works, and the NLR abundances are obtained on the assumption that they can be extrapolated from those of the H ii regions. The calibrations work very well for the Seyfert galaxies, yielding abundance values that agree with those obtained from the H ii regions, and can thus be used for quick estimates of the chemical abundances of the NLRs. For the LINERs, the calibrations yield lower values than those derived from the corresponding H ii regions, suggesting that the assumptions of the models do not apply for them and that there are different physical processes at work in the NLR of the LINERs.
- Publication:
-
The Astronomical Journal
- Pub Date:
- March 1998
- DOI:
- 10.1086/300242
- arXiv:
- arXiv:astro-ph/9711302
- Bibcode:
- 1998AJ....115..909S
- Keywords:
-
- GALAXIES: ISM;
- GALAXIES: ACTIVE;
- GALAXIES: NUCLEI;
- GALAXIES: SEYFERT;
- Astrophysics
- E-Print:
- 14 pages, 4 figures, accepted for publication in The Astronomical Journal