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 AGN isaddressed by introducing new metallicity measurements (based on the method ofHamann & 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 lawwith an index of ~0.2, NLS1s deviate significantly from this relationship atlow luminosities. Adopting the HF93 method based on the N V/C IV line ratio, wefind that NLS1 metallicities are similar to those of some high-redshifthigh-luminosity quasars. We also examined the N IV]/C IV line ratio andcompared it with N V/C IV in a sample of 30 sources including several NLS1s. Wefind that the two do not give a consistent answer regarding the N/C abundanceratio. This result is marginal because of the quality of the data. We suggesttwo alternative explanations to these results: 1) The HF93metallicity--luminosity dependence is not a simple two-parameter dependence andthere is an additional hidden variable in this relationship that has not yetbeen discovered. The additional parameter may be the accretion rate, the age ofthe central stellar cluster or, perhaps, something else. 2) The strong lineratios involving N V 1240A suggested by HF93 are not adequate metallicityindicators for NLS1s and perhaps also other AGN for reasons that are not yetfully understood.Comment: 4 pages, 2 figures; Figure 2 and Tables 1 and 2 were revised. This did not affect any of the main results of the Letter (see also the Erratum: 2002, ApJ, 569, L59