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 NarrowLine Region (NLR) of Seyfert 1 and Seyfert 2 galaxies. It includes a literaturecompilation of the emission line fluxes [OII]3727A, [NeIII]3869A, [OIII]5007Aand [NeV]3426A, as well as 60mum continuum flux, for a sample of 52 Seyfert 1'sand 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 statisticallydifferent, in the sense that Seyfert 1's have values smaller than those ofSeyfert 2's, indicating a higher excitation spectrum. These and other emissionline ratios are compared with sequences of models which combine differentproportions of matter and ionization bounded clouds and also sequences ofmodels which vary only the ionization parameter. This comparison shows that theformer models reproduce better the overall distribution of emission lineratios, indicating that Seyfert 1's have a smaller number of ionization boundedclouds than Seyfert 2's. This difference, together with other results availablein the literature, are interpreted from the point of view of four differentscenarios. The most likely scenario assumes that Seyfert 1's have NLR's smallerthan those of Seyfert 2's, possibly due to a preferential alignment of thetorus axis close to the host galaxy plane axis in Seyfert 1's.Comment: 13 pages, 9 postscript figures, 3 tables, accepted for publication in the Astrophysical Journa