2012ApJ...754...18R

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 ~ 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 (Φ_H< 10¹⁸/cm2/s) and high gas densities (n_H> 10¹²/cm3) 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.