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3 (their ?gure 4). The ?rst (?gure 3a) is where the clouds are separate, with inner clouds being M-clouds, and the outer are I-clouds, while the second (?gure 3b) shows composite clouds, with an I-cloud core and a M-cloud photoevaporating halo. These models reproduce the strong lines quite well as shown in ?gure

2 by the AM/I marked curve, and they also reproduce very well strong high ionization ratios such as He IIλ4686/Hβ (see e.g. ?g. 7, Binette et al. 1996) 3.6. Local Optimally Emitting Clouds The Locally Optimally-emitting Cloud model of Ferguson et al. (1997) is an extension of a model for the BLR. It uses the fact that the spectra we observe in AGN NLRs are likely to be dominated by selection e?ects: each line we observed arises from the clouds in which it is most strongly emitted. As each line emits strongest near its critical density, this recreates the linewidth-critical density relation observed in some NLR. For their model, Ferguson et al. run a grid of simple photoionization models covering a range of densities and radii from the nucleus (connected to incident ?ux). The total line ?ux is then the integral of these over the NLR cloud distribution function, ψ, Fline ∝ r2 Fmodel(r, n)ψ(r, n)dn dr. (2)

1 Matter bounded clouds have low enough column densities that not all ionizing radiation is absorbed. This compares to Ionization bounded clouds where all ionizing radiation is a........