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

Abstract

This paper presents a comparative study of emission line ratios of the Narrow Line Region (NLR) of Seyfert 1 and Seyfert 2 galaxies. It includes a literature compilation of the emission line fluxes [OII]3727A, [NeIII]3869A, [OIII]5007A and [NeV]3426A, as well as 60mum continuum flux, for a sample of 52 Seyfert 1's and 68 Seyfert 2's. The distribution of the emission line ratios [OII]/[NeIII] and [OII]/[NeV] shows that Seyfert 1's and Seyfert 2's are statistically different, in the sense that Seyfert 1's have values smaller than those of Seyfert 2's, indicating a higher excitation spectrum. These and other emission line ratios are compared with sequences of models which combine different proportions of matter and ionization bounded clouds and also sequences of models which vary only the ionization parameter. This comparison shows that the former models reproduce better the overall distribution of emission line ratios, indicating that Seyfert 1's have a smaller number of ionization bounded clouds than Seyfert 2's. 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's have NLR's smaller than those of Seyfert 2's, possibly due to a preferential alignment of the torus axis close to the host galaxy plane axis in Seyfert 1's.