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

This paper presents a comparative study of emission-line ratios of the narr ow-line regions (NLRs) of Seyfert 1 and Seyfert 2 galaxies. It includes a l iterature compilation of the emission-line fluxes [O II] lambda 3727, [Ne I II] lambda 3869, [O III] lambda 5007, and [Ne V] lambda 3426 as well as 60 mu m 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 differe nt: Seyfert 1 galaxies have smaller values than Seyfert 2 galaxies, indicat ing a higher excitation spectrum. These and other emission-line ratios are compared with sequences of models that combine different proportions of mat ter and ionization-bounded clouds and also with sequences of models that va ry only the ionization parameter. This comparison shows that the former mod els better reproduce the overall distribution of emission-line ratios, indi cating that Seyfert I galaxies have a smaller number of ionization-bounded clouds than Seyfert 2 galaxies. This difference, together with other result s available in the literature, are interpreted from the point of view of fo ur different scenarios. The most likely scenario assumes that Seyfert 1 gal axies have smaller NLRs than Seyfert 2 galaxies, possibly due to a preferen tial alignment of the torus axis close to the host galaxy plane axis in Sey fert 1 galaxies.