High-ionization emission line ratios from quasar broad line regions:
  metallicity or density?
  The flux ratios of high-ionization lines are commonly assumed to indicate the
metallicity of the broad emission line region in luminous quasars. When
accounting for the variation in their kinematic profiles, we show that the
NV/CIV, (SiIV+OIV])/CIV and NV/Lya line ratios do not vary as a function of the
quasar continuum luminosity, black hole mass, or accretion rate. Using
photoionization models from CLOUDY , we further show that the observed changes
in these line ratios can be explained by emission from gas with solar
abundances, if the physical conditions of the emitting gas are allowed to vary
over a broad range of densities and ionizing fluxes. The diversity of broad
line emission in quasar spectra can be explained by a model with emission from
two kinematically distinct regions, where the line ratios suggest that these
regions have either very different metallicity or density. Both simplicity and
current galaxy evolution models suggest that near-solar abundances, with parts
of the spectrum forming in high-density clouds, are more likely. Within this
paradigm, objects with stronger outflow signatures show stronger emission from
gas which is denser and located closer to the ionizing source, at radii
consistent with simulations of line-driven disc-winds. Studies using broad-line
ratios to infer chemical enrichment histories should consider changes in
density and ionizing flux before estimating metallicities.
