Gas Metallicity of Narrow‐Line Regions in Narrow‐Line Seyfert 1 Galaxies and Broad‐Line Seyfert 1 Galaxies

Abstract

We investigate gas metallicity of narrow-line regions in narrow-line Seyfert 1 galaxies (NLS1's) and broad-line ones (BLS1's) in order to examine whether or not there is a difference in the gas metallicity between the two populations of Seyfert 1 galaxies. We apply two methods to study this issue. One is to use the emission-line flux ratio [N II] λ6583/Hαnarrow in combination with some other optical emission line flux ratios. This method, which has been often applied to Seyfert 2 galaxies, suggests that the gas metallicity of narrow-line regions is indistinguishable, or possibly higher in BLS1's than in NLS1's. On the contrary, the other method, in which only forbidden emission line fluxes are used, results in that NLS1's tend to possess metal-richer gas in the narrow-line regions than BLS1's. We point out that this inconsistency may be owing to the contamination of the narrow component of permitted lines by the broad component in the first method. Since the results derived by using only forbidden emission line fluxes do not suffer from any uncertainty of the fitting function for the broad component of Balmer lines, the results from this method are more reliable than those derived by using permitted lines. We thus conclude that the gas metallicity of narrow-line regions tends to be higher in NLS1's than in BLS1's.