2012ApJ...757...62N

We describe a method for estimating physical conditions in the broad-line region (BLR) for a significant subsample of Seyfert 1 nuclei and quasars. Several diagnostic ratios based on intermediate (Al III λ1860, Si III] λ1892) and high (C IV λ1549, Si IV λ1397) ionization lines in the UV spectra of quasars are used to constrain density, ionization, and metallicity of the emitting gas. We apply the method to two extreme Population A quasars–the prototypical NLSy1 I Zw 1 and higher z source SDSS J120144.36+011611.6. Under assumptions of spherical symmetry and pure photoionization we infer BLR physical conditions: low ionization (ionization parameter <10^–2), high density (10¹²-10¹³/cm3), and significant metal enrichment. Ionization parameter and density can be derived independently for each source with an uncertainty that is less than ±0.3 dex. We use the product of density and ionization parameter to estimate the BLR radius and derive an estimation of the virial black hole mass (M_BH). Estimates of M_BH based on the "photoionization" analysis described in this paper are probably more accurate than those derived from the mass-luminosity correlations widely employed to compute black hole masses for high-redshift quasars.