2005MNRAS.356.1029B

The high-ionization lines in active galactic nuclei (AGN), such as Civ, 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 Civ suggest that non-gravitational effects, such as obscuration and radiation pressure, may affect the line profile. We explore the relation between the Hβ and Civ 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 Civ lines (full width at half-maximum, FWHM < 2000 km/s) 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 the Civ line is broader than Hβ, but the reverse is true when the Hβ FWHM > 4000 km/s. This argues against the view that Civ generally originates closer to the centre, compared with Hβ. (3) Civ 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 Civ is strongly blueshifted and asymmetric have a high L/L_Edd, but the reverse is not true. This suggests that a high L/L_Eddis a necessary but not sufficient condition for generating a blueshifted asymmetric Civ 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 Civ emission, stronger Civ absorption and a higher optical continuum polarization.