The Civ line width distribution for quasars and its implications for
  broad-line region dynamics and virial mass estimation
  We perform an extensive analysis of the Civ 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 develop a line fitting procedure
and apply it to the Civ line in spectra from the SDSS, 2QZ and 2SLAQ surveys.
  Our results are compared with a previous study of the Mgii line in the same
sample. Civ tends to be broader than the Mgii line in spectra that have both,
and the average ratio between the lines is consistent with a simplistic model
for a photoionised, virialised and stratified broad-line region. There is 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 line widths.
  Motivated by previous work, we examine the dispersion in the distribution of
Civ line widths. We find that the dispersion in Civ line widths is essentially
independent of both redshift and luminosity. This is in stark contrast to the
Mgii line, which shows a strong luminosity dependence.
  Finally we consider our results in terms of their implications for virial
black hole mass estimation. The inconsistency between Mgii and Civ line widths
in single spectra, combined with the differing behaviour of the Mgii and Civ
line width distributions, indicates that there must be an inconsistency between
Mgii and Civ virial mass estimators. Furthermore, the level of intrinsic
dispersion in Mgii and Civ line widths contributes less dynamic range to virial
mass estimates than the error associated with the estimates. The indication is
that the line width term in these UV virial mass estimators may be essentially
irrelevant with respect to the typical uncertainty on a mass estimate.
