The narrow-line region (NLR) in AGNs is of interest for at least three interrelated reasons. First, the NLR is the largest spatial scale where the ionizing radiation from the central source dominates over other sources. Second, the NLR is the only AGN component which is spatially resolved in the optical – this is of particular importance as the NLR is clearly illuminated in a non-isotropic manner by the central source. Finally, the NLR dynamics might tell us something about how AGNs are fueled.

Narrow-Line Spectra

As in the case of the BLR, the relative strengths of the emission lines we observe in NLR spectra allow us to discern some of the properties of the ionizing spectrum. Unlike the BLR, the electron densities in the NLR are low enough that many forbidden transitions are not collisionally suppressed. This allows us to use the intensity ratios of certain pairs of forbidden lines to measure the electron densities and temperatures in the NLR gas (§6.2). In comparison to the BLR, the analysis is simplified by the low densities. On the other hand, however, in the case of the NLR an additional complication is introduced into the spectroscopic analysis by significant amounts of dust since the NLR arises outside the dust sublimation radius (eq. 4.15); indeed, it may well be that the radius where dust sublimates provides the fundamental demarcation between the BLR and the NLR.