Is there a metallicity-luminosity relationship in active galactic nuclei? The case of narrow-line Seyfert 1 galaxies

The well-known relationship between metallicity and luminosity in active galactic nuclei (AGNs) is addressed by introducing new metallicity measurements (based on the method of Hamann & Ferland, hereafter HF) for a sample of narrow-line Seyfert 1 galaxies (NLS1s). Our new results, based on a sample of 162 AGNs, including nine NLS1s, indicate that while broad-line AGNs trace a metallicity-luminosity power law with an index of similar to0.2, NLS1s deviate significantly from this relationship at low luminosities. Adopting the HF method based on the N v/C IV line ratio, we find that NLS1 metallicities are similar to those of some high-redshift, high-luminosity quasars. We also examined the N IV]/C IV line ratio and compared it with N v/C IV in a sample of 30 sources including several NLS1s. We find that the two do not give a consistent answer regarding the N/C abundance ratio. This result is marginal because of the quality of the data. We suggest two alternative explanations to these results: (1) The HF metallicity-luminosity dependence is not a simple two-parameter dependence, and there is an additional hidden variable in this relationship that has not yet been discovered. The additional parameter may be the accretion rate, the age of the central stellar cluster, or perhaps something else. (2) The strong line ratios involving N v lambda1240 suggested by HF are not adequate metallicity indicators for NLS1s and perhaps also for other AGNs for reasons that are not yet fully understood.