Ionized gas and dust in active galactic nuclei

Abstract

Lick Observatory spectrophotometric results on the emission-line spectra of Seyfert and radio galaxies are reviewed and discussed. The spectra of Seyfert 2 and narrow-line radio galaxies are similar, and are in qualitative agreement with the idea that they are photoionized by a input radiation field extending to high energies. Their measured Balmer-line ratios agree with recombination theory, modified by the effects of normal interstellar extinction. The spectra of Seyfert 1 and broad-line radio galaxies are to a first approximation similar to each other, but statistically can be distinguished. Several Seyfert galaxies have spectra intermediate between Seyfert 1 and 2. The broad emission lines are emitted in a small, dense region containing gas with a wide range in velocity. The broad Balmer-line intensity ratios are affected by high density. The evidence for dust in the high-density region is not conclusive, but it is probably present in at least some Seyfert 1 galaxies. Seyfert 1, 1.5, and 2 galaxies have similar distribution functions of emission-line H..beta.. equivalent widths, suggesting that photoionization is the energy input to the broad-line dense gas as well as to the narrow-line gas. A strong narrow emission-line component is statistically correlated with radio emission. A cylindrical model withmore » strong density condensations fits many of the observational data.« less