The difference between the narrow-line regions of Seyfert 1 and Seyfert 2 galaxies

Abstract

This paper presents a comparative study of emission-line ratios of the narrow-line regions (NLRs) of Seyfert 1 and Seyfert 2 galaxies. It includes a literature compilation of the emission-line fluxes [O II] λ3727, [Ne III] λ3869, [O III] λ5007, and [Ne V] λ3426 as well as 60 μm continuum flux, for a sample of 52 Seyfert 1 and 68 Seyfert 2 galaxies. The distribution of the emission-line ratios [O II]/[Ne III] and [O II]/[Ne V] shows that Seyfert 1 and Seyfert 2 galaxies are statistically different : Seyfert 1 galaxies have smaller values than Seyfert 2 galaxies, indicating a higher excitation spectrum. These and other emission-line ratios are compared with sequences of models that combine different proportions of matter and ionization-bounded clouds and also with sequences of models that vary only the ionization parameter. This comparison shows that the former models better reproduce the overall distribution of emission-line ratios, indicating that Seyfert 1 galaxies have a smaller number of ionization-bounded clouds than Seyfert 2 galaxies. This difference, together with other results available in the literature, are interpreted from the point of view of four different scenarios. The most likely scenario assumes that Seyfert 1 galaxies have smaller NLRs than Seyfert 2 galaxies, possibly due to a preferential alignment of the torus axis close to the host galaxy plane axis in Seyfert 1 galaxies.