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ABSTRACT 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 N v/C iv, (Si iv + O iv])/C iv, and N v/Ly α 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 that 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. 

Abstract We examine the UV/X-ray properties of 1378 quasars in order to link empirical correlations to theoretical models of the physical mechanisms dominating quasars as a function of mass and accretion rate. The clarity of these correlations is improved when (1) using Civbroad emission line equivalent width (EQW) and blueshift (relative to systemic) values calculated from high signal-to-noise ratio reconstructions of optical/UV spectra and (2) removing quasars expected to be absorbed based on their UV/X-ray spectral slopes. In addition to using the traditional Civparameter space measures of CivEQW and blueshift, we define a “Civ∥ distance” along a best-fit polynomial curve that incorporates information from both Civparameters. We find that the Civ∥ distance is linearly correlated with both the optical-to-X-ray slope,α_ox, and broad-line HeiiEQW, which are known spectral energy distribution indicators, but does not require X-ray or high spectral resolution UV observations to compute. The Civ∥ distance may be a better indicator of the mass-weighted accretion rate, parameterized byL/L_Edd, than the CivEQW or blueshift alone, as those relationships are known to break down at the extrema. Conversely, there is only a weak correlation with the X-ray energy index (Γ), an alternateL/L_Eddindicator. We find no X-ray or optical trends in the direction perpendicular to the Civdistance that could be used to reveal differences in accretion disk, wind, or corona structure that could be widening the CivEQW–blueshift distribution. A different parameter (such as metallicity) not traced by these data must come into play. 

Abstract Recent improvements to atomic energy-level data allow, for the first time, accurate predictions to be made for the Fe iii line emission strengths in the spectra of luminous, $$L_\text{bol}\simeq 10^{46}-10^{48}{{\rm \, erg}{\rm \, s}^{-1}\,}$$, Active Galactic Nuclei. The Fe iii emitting gas must be primarily photoionized, consistent with observations of line reverberation. We use Cloudy models exploring a wide range of parameter space, together with ≃26,000 rest-frame ultraviolet spectra from the Sloan Digital Sky Survey, to constrain the physical conditions of the line emitting gas. The observed Fe iii emission is best accounted for by dense (nH ≃ 1014 cm−3) gas which is microturbulent, leading to smaller line optical depths and fluorescent excitation. Such high density gas appears to be present in the central regions of the majority of luminous quasars. Using our favoured model, we present theoretical predictions for the relative strengths of the Fe iii UV34 λλ1895,1914,1926 multiplet. This multiplet is blended with the Si iii] λ1892 and C iii] λ1909 emission lines and an accurate subtraction of UV34 is essential when using these lines to infer information about the physics of the broad line region in quasars.