Knowledge Bank

The rapid response of the broad emission lines to continuum variations in the Seyfert galaxy NGC 5548 implies that line-emitting regions are very close to the continuum source. As described in this contribution, photoionization models must be modified to deal with the very high radiation densities in the broad-line region. An immediate consequence of the high radiation density is that the emission-line cloud column densities must be large in order for the clouds to remain optically thick in the ionizing continuum and hence show recombination line variability. This leads to significant optical depths in the lines as well, with the result that line emission is radiated anisotropically, with most of the radiation emitted from the inner, irradiated face of the cloud. This may account for the apparent absence of line-emitting material along our line of sight to the continuum source, as inferred from reverberation studies. We show that the observed continuum-Hß transfer function for NGC 5548 is consistent with a spherical distribution of anisotropically radiating clouds. We estimate the density in the Lyα-C IV-emitting regions as N ≈ 10^11 cm^-3, largely due to the lack of significant C III] λ1909 emission from this gas. The ionization parameter is then set by the source-cloud separation. The clouds are predicted to be significant emitters of O v λ1218, which would cause Lyα to appear redshifted and respond in a nonlinear manner. We show that clouds at slightly higher densities but cosmic abundances can be strong emitters of lines such as Al III λ1860, largely because cooling by other lines is suppressed by line thermalization at these densities.