On the central abundances of Active Galactic Nuclei and Star-forming Galaxies

TL;DR

This study compares oxygen abundance estimates in AGNs and star-forming galaxies using different methods, revealing a significant underestimation by the Te-method and suggesting no extraordinary enrichment in AGN NLRs.

Contribution

It demonstrates the discrepancy between the Te-method and strong-line method in abundance estimates and clarifies the chemical enrichment status of AGN NLRs.

Findings

Te-method underestimates oxygen abundance by up to 2 dex.

Strong-line method yields higher, more consistent abundances.

NLR oxygen abundance is below the maximum galaxy enrichment level.

Abstract

We examine the relation between oxygen abundances in the narrow-line regions (NLRs) of active galactic nuclei (AGNs) estimated from the optical emission lines through the strong-line method (the theoretical calibration of Storchi-Bergmann et al.(1998)), via the direct Te-method, and the central intersect abundances in the host galaxies determined from the radial abundance gradients. We found that the Te-method underestimates the oxygen abundances by up to ~2 dex (with average value of ~0.8 dex) compared to the abundances derived through the strong-line method. This confirms the existence of the so-called "temperature problem" in AGNs. We also found that the abundances in the centres of galaxies obtained from their spectra trough the strong-line method are close to or slightly lower than the central intersect abundances estimated from the radial abundance gradient both in AGNs and Star-forming galaxies. The oxygen abundance of the NLR is usually lower than the maximum attainable abundance in galaxies (~2 times the solar value). This suggests that there is no extraordinary chemical enrichment of the NLRs of AGNs.