Results of detailed modeling of the narrow-line region of Seyfert galaxies

We present model line profiles of [O II] lambda 3727, [Ne III] lambda 3869, [O III] lambda 5007, [Fe VII] lambda 6087, [Fe X] lambda 6374, [O I] lambda 6300, H alpha lambda 6563, and [S II] lambda 6731. The profiles presented here illustrate explicitly the pronounced effects that collisional de-excitation, and that spatial variations in both the ionization parameter and cloud column density, have on narrow-line region (NLR) model profiles. The above effects were included only qualitatively in a previous analytical treatment by Moore & Cohen. By making a direct correspondence between these model profiles and the analytical model profiles of Moore & Cohen, and by comparing with the observed profiles presented in a companion paper and also with those presented elsewhere in the literature, we strengthen some of the conclusions of Moore & Cohen. Most notably, we argue for constant ionization parameter, uniformly accelerated outflow of clouds that are individually stratified in ionization, and the interpretation of emission-line width correlations with ionization potential as a column density effect. For comparison with previous observational studies, such as our own in a companion paper, we also calculate profile parameters for some of the models, and we present and discuss the resulting line width correlations with critical density(n(cr)) and ionization potential (IF). Because the models we favor are those that produce extended profile wings as observed in high spectral resolution studies, the line width correlations of our favoured models are of particular interest. Line width correlations with n(cr) and/or IP result only if the width parameter is more sensitive to extended profile wings than is the full width at half-maximum (FWHM). Correlations between FWHM and n(cr) and/or IP result only after convolving the model profiles with a broad instrumental profile that simulates the lower spectral resolution used in early observational studies. The model in agreement with the greatest number of observational considerations has electron density decreasing outward from n(e) approximate to 10(6) cm(-3) to n(e) approximate to 10(2) cm(-3) and, due to collisional de-excitation effects in the lowest velocity clouds, it generates broad flat-topped profile peaks in the lines of lowest critical density (e.g., [O II] lambda 3727 and [S II] lambda lambda 6716, 6731). Because the observed profile peaks of both low and high critical density lines are often very similar, our favored model requires a contribution to NLR emission-line spectra from low-velocity, low-density, and low-ionization gas not included in the model NLR.