Is There a Metallicity--Luminosity Relationship in AGN? The Case of Narrow-Line Seyfert 1 Galaxies

The well known relationship between metallicity and luminosity in AGN is addressed by introducing new metallicity measurements (based on the method of Hamann & Ferland 1993; HF93) for a sample of narrow-line Seyfert 1 (NLS1) galaxies. Our new results, based on a sample of 162 AGN, including 9 NLS1s, indicate that, while broad-line AGN trace a metallicity--luminosity power law with an index of ~0.2, NLS1s deviate significantly from this relationship at low luminosities. Adopting the HF93 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 HF93 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 1240A suggested by HF93 are not adequate metallicity indicators for NLS1s and perhaps also other AGN for reasons that are not yet fully understood.