New Constraints on the Quasar Broad Emission Line Region

We demonstrate a new technique for determining the physical conditions of the broad-line-emitting gas in quasars, using near-infrared hydrogen emission lines. Unlike higher ionization species, hydrogen is an efficient line emitter for a very wide range of photoionization conditions, and the observed line ratios depend strongly on the density and photoionization state of the gas present. A locally optimally emitting cloud model of the broad emission line region was compared to measured emission lines of four nearby (z approximate to 0.2) quasars that have optical and NIR spectra of sufficient signal to noise to measure their Paschen lines. The model provides a good fit to three of the objects, and a fair fit to the fourth object, an ultraluminous infrared galaxy. We find that low-incident-ionizing fluxes (Phi(H) < 10(18) cm(-2) s(-1)) and high gas densities (n(H) > 10(12) cm(-3)) are required to reproduce the observed hydrogen emission line ratios. This analysis demonstrates that the use of composite spectra in photoionization modeling is inappropriate; models must be fitted to the individual spectra of quasars.