Physical state of the emission-line region

The physical properties of the ionized gas in the active nuclei of Seyfert and radio galaxies derived from their emission-line spectra, are reviewed. Mean densities and temperatures in Seyfert 2 and narrow-line radio galaxies are in the ranges < 103 - 104 cm-3 and 10 0000 - 20 0000 respectively, while in the narrow-line regions of Seyfert 1 and broadline and radio galaxies the temperatures are similar but the densities range up to 106 cm-3. In the broad-line regions the densities are 108 cm-3 or greater. The abundances of the elements seem more or less normal. The relative strengths of the lines approximately match photoionization models with a power-law input spectrum, including the presence of high-ionization with lines such as [Fe X] and [Fe XI] along with great strengths of the neutral and low-ionization lines such as [O I], [N I] and [S II]. The Fe II emission which occurs in most Seyfert 1 galaxies but is much weaker or absent in broad-line radio galaxies occurs in the broad-line high-density region. Its excitation has been attributed to resonance fluorescence; the mechanism is at present being studied in detail observationally. The internal velocities observed in active nuclei are largely due to mass motions. The masses of ionized gas in the dense high-velocity regions are relatively small, and the structure of the gas must be highly filamentary or inhomogeneous. Dust is an important constituent of the active nuclei. (Auth.)