Studies of narrow emission lines in AGNs

Optical spectra having moderately high resolution (~ 2 — 5 Å) are being used to study the profiles of narrow emission lines in active galactic nuclei (AGNs). It is often found that forbidden lines associated with high critical densities for collisional deexcitation are the broadest. A good example is [O III] λ4363 [ n e (crit) ≈ 3 × 10 7 cm −3 ], whose width can be more than twice that of [O III] λ5007 [ n e (crit) ≈ 8 × 10 5 cm −3 ]. The tight correlation between line width and n e (crit) implies that a much larger range of densities (∼ 10 2 — 10 7 cm −3 ) must be present among clouds in the narrow-line region than was previously believed. At times there almost appears to be a continuity between the narrow- and broad-line regions. In some objects the dense, high-velocity clouds are optically thick to ionizing radiation, since they emit [O I] λ6300 as well as species of much higher ionization (such as [Ne V] λ3426). These results help eliminate several difficulties in photoionization models of LINERs. It may also be possible to use the observed line widths as probes of the gravitational potential in AGNs.