The high‐ionization lines in active galactic nuclei (AGN), such as C iv, tend to be blueshifted with respect to the lower‐ionization lines, such as Hβ, and often show a strong blue excess asymmetry not seen in the low‐ionization lines. There is accumulating evidence that the Hβ profile is dominated by gravity, and thus provides a useful estimate of the black hole mass in AGN. The shift and asymmetry commonly seen in C iv suggest that non‐gravitational effects, such as obscuration and radiation pressure, may affect the line profile. We explore the relation between the Hβ and C iv profiles using the ultraviolet (UV) spectra available for 81 of the 87 z≤ 0.5 PG quasars in the Boroson & Green sample. We find the following. (1) Narrow C iv lines (full width at half‐maximum, FWHM < 2000 km s−1) are rare (∼2 per cent occurrence rate) compared with narrow Hβ lines (∼20 per cent). (2) In most objects where the Hβ FWHM < 4000 km s−1 the C iv line is broader than Hβ, but the reverse is true when the Hβ FWHM > 4000 km s−1. This argues against the view that C iv generally originates closer to the centre, compared with Hβ. (3) C iv appears to provide a significantly less accurate, and possibly biased estimate of the black hole mass in AGN, compared with Hβ. (4) All objects where C iv is strongly blueshifted and asymmetric have a high L/LEdd, but the reverse is not true. This suggests that a high L/LEdd is a necessary but not sufficient condition for generating a blueshifted asymmetric C iv emission. (5) We also find indications for dust reddening and scattering in ‘normal’ AGN. In particular, PG quasars with a redder optical–UV continuum slope show weaker C iv emission, stronger C iv absorption and a higher optical continuum polarization.

The Broad Line Region (BLR) in AGN is composed of dense gas (∼ 10 11 cm −3 ) on sub-pc scale, which absorbs about 30 per cent of the ionising continuum. The outer size of the BLR is likely set by dust sublimation, and its density by the incident radiation pressure compression (RPC). But, what is the origin of this gas, and what sets its covering factor (CF)? Czerny & Hryniewicz (2011) suggested that the BLR is a failed dusty wind from the outer accretion disc. We explore the expected dust properties, and the implied BLR structure. We find that graphite grains sublimate only at T ≃ 2000 K at the predicted density of ∼ 10 11 cm −3 , and therefore large graphite grains (≥ 0.3 µm) survive down to the observed size of the BLR, R BLR . The dust opacity in the accretion disc atmosphere is ∼ 50 times larger than previously assumed, and leads to an inflated torus-like structure, with a predicted peak height at R BLR . The illuminated surface of this torus-like structure is a natural place for the BLR. The BLR CF is mostly set by the gas metallicity, the radiative accretion efficiency, a dynamic configuration, and ablation by the incident optical-UV continuum. This model predicts that the BLR should extend inwards of R BLR to the disc radius where the surface temperature is ≃ 2000 K, which occurs at R in ≃ 0.18R BLR . The value of R in can be tested by reverberation mapping of the higher ionisation lines, predicted by RPC to peak well inside R BLR . The dust inflated disc scenario can also be tested based on the predicted response of R BLR and the CF to changes in the AGN luminosity and accretion rate.

The origin of the luminosity dependence of the equivalent width (EW) of broad emission lines in active galactic nuclei (AGN) – the Baldwin effect – is not firmly established yet. We explore this question for the broad C ivλ1549 line using the Boroson & Green sample of the 87 z≤ 0.5 Bright Quasar Survey (BQS) quasars. Useful UV spectra of the C iv region are available for 81 of the objects, which are used to explore the dependence of the C iv EW on various emission properties. We confirm earlier results on the strong correlations of the C iv EW with some of the emission parameters which define the Boroson & Green Eigenvector 1, and with the optical to X‐ray slope αox. In addition, we find a strong correlation of the C iv EW with the relative accretion rate, L/LEdd. Since L/LEdd drives some of the Eigenvector 1 correlations, it may be the primary physical parameter which drives the Baldwin effect for C iv.

Active galactic nuclei (AGN) are characterized by similar broad emission lines properties at all luminosities (10 39 -10 47 erg s −1 ). What produces this similarity over a vast range of 10 8 in luminosity? Photoionization is inevitably associated with momentum transfer to the photoionized gas. Yet, most of the photoionized gas in the Broad Line Region (BLR) follows Keplerian orbits, which suggests that the BLR originates from gas with a large enough column for gravity to dominate. The photoionized surface layer of the gas must develop a pressure gradient due to the incident radiation force. We present solutions for the structure of such a hydrostatic photoionized gas layer in the BLR. The gas is stratified, with a low-density highly-ionized surface layer, a density rise inwards, and a uniform-density cooler inner region, where the gas pressure reaches the incident radiation pressure. This radiation pressure confinement (RPC) of the photoionized layer leads to a universal ionization parameter U ∼ 0.1 in the inner photoionized layer, independent of luminosity and distance. Thus, RPC appears to explain the universality of the BLR properties in AGN. We present predictions for the BLR emission per unit covering factor, as a function of distance from the ionizing source, for a range of ionizing continuum slopes and gas metallicity. The predicted mean strength of most lines (excluding H β), and their different average-emission radii, are consistent with the available observations.

Active galactic nuclei (AGN) display an extreme range in the narrow emission‐line equivalent widths. Specifically, in the Palomar–Green (PG) quasar sample, the equivalent width of the narrow [O iii]λ5007 line has a range of >300 (<0.5 to 157 Å), while the broad Hβ line, for example, has a range of 10 only (23 to 230 Å). The strength of [O iii]λ5007 is modulated by the covering factor (CF) of the narrow‐line region (NLR) gas, its density (ne) and ionization parameter (U). To explore which of these factors produces the observed large range in [O iii]λ5007 strength, we measure the strength of the matching narrow Hβ and [O iii]λ4363 lines, detected in 40 out of the 87 PG quasars with z < 0.5 in the Boroson & Green sample. The photoionization code cloudy is then used to infer CF, ne and U in each object, assuming a single uniform emitting zone. We find that the range of CF (∼0.02–0.2) contributes about twice as much as the range in both ne and U towards modulating the strength of the [O iii]λ5007 line. The CF is inversely correlated with luminosity, but it is not correlated with L/LEdd as previously speculated. The single‐zone [O iii]λ5007 emitting region is rather compact, having RNLR= 40L0.4544 pc. These emission lines can also be fitted with an extreme two‐zone model, where [O iii]λ4363 is mostly emitted by a dense (ne= 107 cm−3) inner zone at RinNLR=L0.544 pc, and [O iii]λ5007 by a low‐density (ne= 103 cm−3) extended outer zone at RoutNLR= 750L0.3444 pc. Such an extended [O iii]λ5007 emission should be well resolved by Hubble Space Telescope imaging of luminous AGN. Further constraints on the radial gas distribution in the NLR can be obtained from the spectral shape of the infrared continuum emitted by the associated dust.