SIMBAD references

We perform an extensive analysis of the Civλ1549 line in three large spectroscopic surveys of quasars. Differing approaches for fitting the Civ line can be found in the literature, and we compare the most common methods to highlight the relative systematics associated with each. We choose the line-fitting procedure that results in a symmetric profile for the Civ line and gives accurate fits to local emission features around the line, and use this approach to measure the width of the Civ line in spectra from the Sloan Digital Sky Survey (SDSS) and 2dF QSO Redshift (2QZ) and 2dF SDSS LRG And QSO (2SLAQ) surveys.

The results are compared with a previous study of the Mgiiλ2799 line in the same sample. We find the Civ line tends to be broader than the Mgii line in spectra that have both lines, and the average ratio between the lines is consistent with a simplistic model for a photoionized, virialized and stratified broad-line region. There exists a statistically significant correlation between the widths of the Civ and Mgii lines. However, the correlation is weak, and the scatter around a best fit is only marginally less than the full dynamic range of linewidths.

Motivated by previous work on the Mgii line, we examine the dispersion in the distribution of Civ linewidths. We find that the dispersion in Civ linewidths is essentially independent of both redshift and luminosity. This result is in stark contrast to the Mgii line, which shows a strong luminosity dependence. Furthermore, we demonstrate that the low level of dispersion in Civ linewidth (∼0.08dex) is inconsistent with a pure-disc model for the emitting region and use our data to constrain simple models for the broad-line region.

Finally, we consider our results in terms of their implications for the virial technique for estimating black hole masses. The inconsistency between Mgii and Civ linewidths in single spectra, combined with the differing behaviour of the Mgii and Civ linewidth distributions as a whole, indicates that there must be an inconsistency between Mgii and Civ virial mass estimators. Furthermore, the level of intrinsic dispersion in Mgii and Civ linewidths contributes less dynamic range to virial mass estimates than the error associated with the estimates. The indication is that the linewidth term in these UV virial mass estimators may be essentially irrelevant with respect to the typical uncertainty on a mass estimate.