What controls the C iv line profile in active galactic nuclei?

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 / L Edd , but the reverse is not true. This suggests that a high L / L Edd 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.