On the Effects of Dissipative Turbulence on the Narrow Emission‐Line Ratios in Seyfert Galaxies

Abstract

We present a photoionization model study of the effects of microturbulence and dissipative heating on emission lines for number and column densities, elemental abundances, and ionizations typical for the narrow emission line regions (NLRs) of Seyfert galaxies. Earlier studies of NLR spectra generally found good agreement between the observations and the model predictions for most strong emission lines, such as [O III] λ5007, [O II] λ3727, [N II] λ6583, [Ne III] λ3869, and the H and He recombination lines. Nevertheless, the strengths of lines from species with ionization potentials greater than that of He+ (54.4 eV), e.g., N+4 and Ne+4, were often underpredicted. Among the explanations suggested for these discrepancies were (selectively) enhanced elemental abundances and contributions from shock-heated gas. Interestingly, the NLR lines have widths of several 100 km s-1, well in excess of the thermal broadening. If this is due to microturbulence, and the turbulence dissipates within the emission-line gas, the gas can be heated in excess of that due to photoionization. We show that the combined effects of turbulence and dissipative heating can strongly enhance N V λ1240 (relative to He II λ1640), while the heating alone can boost the strength of [Ne V] λ3426. We suggest that this effect is present in the NLR, particularly within ~100 pc of the central engine. Finally, since microturbulence would make clouds robust against instabilities generated during acceleration, it is not likely to be a coincidence that the radially outflowing emission-line gas is turbulent.