The C IV linewidth distribution for quasars and its implications for broad-line region dynamics and virial mass estimation

We perform an extensive analysis of the C IV lambda 1549 line in three large spectroscopic surveys of quasars. Differing approaches for fitting the C IV 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 C IV line and gives accurate fits to local emission features around the line, and use this approach to measure the width of the C IV 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 Mg II lambda 2799 line in the same sample. We find the C IV line tends to be broader than the Mg II 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 C IV and Mg II 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 Mg II line, we examine the dispersion in the distribution of C IV linewidths. We find that the dispersion in C IV linewidths is essentially independent of both redshift and luminosity. This result is in stark contrast to the Mg II line, which shows a strong luminosity dependence. Furthermore, we demonstrate that the low level of dispersion in C IV linewidth (similar to 0.08 dex) 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 Mg II and C IV linewidths in single spectra, combined with the differing behaviour of the Mg II and C IV linewidth distributions as a whole, indicates that there must be an inconsistency between Mg II and C IV virial mass estimators. Furthermore, the level of intrinsic dispersion in Mg II and C IV 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.