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

Abstract

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.