Photoionization Models of the Broad-line Region

The strong broad emission lines in the optical and UV spectra of Active Galactic Nuclei (AGN) are important for several reasons. Not only do they give us information about the structure of the AGN, their properties are now used to estimate black hole masses and metallicities in the vicinity of quasars, and these estimates are propagated widely throughout astronomy today. Photoionization codes such as Cloudy are invaluable for understanding the physical conditions of the gas emitting AGN broad lines. In this review, we discuss briefly the development of the historical ``standard'' model. We then review evidence that the following are important: 1.) the column density, in particular the presence of gas optically thin to the hydrogen continuum, influences the line emission; 2.) the BLR emission region is comprised of gas with a range of densities and ionization parameters; 3.) the spectral energy distribution of an individual AGN influences the line ratios in an observable way.