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 ionisation species, hydrogen is an efficient line emitter
for a very wide range of photoionisation conditions, and the observed line
ratios depend strongly on the density and photoionisation 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 ($z\approx0.2$)
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, a ULIRG. We find that low
incident ionising fluxes ($\phih<10^{18}$\cmsqs), and high gas densities
($\nh>10^{12}$\cmcu) are required to reproduce the observed hydrogen emission
line ratios. This analysis demonstrates that the use of composite spectra in
photoionisation modelling is inappropriate; models must be fitted to the
individual spectra of quasars.
