Despite years of observational and theoretical effort, we cannot yet say much about the BLR with any certainty. We know that it is dense by nebular standards and hot (T ≈ 104 K), and that it is probably some supersonic, fairly continuous flow. The dynamics of the BLR are unknown, though it is clear that the gravitational acceleration caused by the central source is sufficiently dominant that we can use the BLR size and velocity dispersion to estimate the central black hole masses with an accuracy of about a factor of three. We know that it has a stratified ionization structure, and may have multiple kinematic components; currently strong candidates include both disk winds and emission from the extended parts of the accretion disk itself, with the strength of different components varying from object to object and from emission line to emission line. We know that the size of the BLR scales simply with luminosity, both globally from object-to-object and in a single source as the mean luminosity varies with time. The latter in particular argues that there is considerably more mass involved in the BLR than presumed by naive models. We have argued that the best way to determine the structure and kinematics of the BLR is by obtaining velocity-delay maps for multiple emission lines by reverberation mapping, and that the data requirements, while somewhat daunting, are realizable with existing technology.
Acknowledgements
I am grateful for support of this work by the US National Science Foundation through grant AST-0205964 to The Ohio State University. I would like to thank my collaborators and Nahum Arav, Kirk Korista, Thaisa Storchi-Bergmann, and Dan Vanden Berk for assistance with figures. I also thank the organizers for their fine hospitality in Chile. I apologize for the incomplete nature of the reference list; the few selected references are intended to provide the reader an entry point to the extensive literature on the broad-line region rather than a comprehensive bibliography.