Comparing Single-Epoch Virial Black Hole Mass Estimators for Luminous
  Quasars
  Single-epoch virial black hole (BH) mass estimators utilizing broad emission
lines have been routinely applied to high-redshift quasars to estimate their BH
masses. Depending on the redshift, different line estimators (Halpha, Hbeta,
MgII, CIV) are often used with optical/near-infrared spectroscopy. Here we use
a homogeneous sample of 60 intermediate-redshift (z~1.5-2.2) SDSS quasars with
optical and near-infrared spectra covering CIV through Halpha to investigate
the consistency between different line estimators. We critically compare
restframe UV line estimators (CIV, CIII], and MgII) with optical estimators
(Hbeta and Halpha) in terms of correlations between line widths and between
continuum/line luminosities, for the high-luminosity regime (L_5100>10^45.4
erg/s) probed by our sample. The continuum luminosities of L_1350 and L_3000,
and the broad line luminosities are well correlated with L_5100. We found that
the MgII FWHM correlates well with the FWHMs of the Balmer lines, and that the
MgII line estimator can be calibrated to yield consistent virial mass estimates
with those based on the Hbeta/Halpha estimators, thus extending earlier results
on less luminous objects. The CIV FWHM is poorly correlated with the Balmer
line FWHMs, and the scatter between the CIV and Hbeta FWHMs consists of an
irreducible part (~0.12 dex), and a part that correlates with the blueshift of
the CIV centroid relative to that of Hbeta. The CIII] FWHM is found to
correlate with the CIV FWHM, and hence is also poorly correlated with the Hbeta
FWHM. While the CIV and CIII] lines can be calibrated to yield consistent
virial mass estimates as Hbeta on average, the scatter is substantially larger
than MgII, and the usage of CIV/CIII] FWHM in the mass estimators does not
improve the agreement with the Hbeta estimator. (Abridged)
