A dynamical model for the narrow line region of Seyfert galaxies

We have computed detailed emission line ratios and profiles for the ‘catapult’ model of the NLR. According to this model the emission lines arise from interstellar clouds interacting with a wind from the central source. For clouds which are ejected by the ram pressure of the wind we find agreement with average Seyfert 2 line ratios for the main diagnostic lines, within the 2σ dispersions. This also gives us the observed constancy of line profiles for different lines. The Seyfert 1 line ratios are obtained by adding a component of clouds which are not ejected. Our predicted line ratios (relative to [O III]λ5007) for Seyfert 1s agree to better than lσ with the observed averages. We obtain a wide range of widths for the various lines, giving a reasonable agreement with the proportionality between widths at 1/2 and 1/4 peak intensity, as noted by De Robertis and Osterbrock. The observed kurtosis is obtained after allowance for instrumental resolution, but the model lines are, in fact, more peaked than is revealed by the resolution of current observations. We show also that the internal cloud structure for most of the diagnostic lines is not important in determining the gross features of the emission. However, the two lines [S II]λ16718 and [S II]λ6733 become stronger by a factor of ∼2 when using exponentially varying pressure inside the clouds rather than constant pressure. We conclude that the sulphur lines, in an isothermal cloud, are mainly produced in low density regions and we therefore propose their use as a diagnostic to investigate the internal cloud structure.