Extending the Calibration of CIV-Based Single-Epoch Black Hole Mass
  Estimators for Active Galactic Nuclei
  We provide an updated calibration of CIV $\lambda1549$ broad emission
line-based single-epoch (SE) black hole (BH) mass estimators for active
galactic nuclei (AGNs) using new data for six reverberation-mapped AGNs at
redshift $z=0.005-0.028$ with BH masses (bolometric luminosities) in the range
$10^{6.5}-10^{7.5}$ $M_{\odot}$ ($10^{41.7}-10^{43.8}$ erg s$^{\rm -1}$). New
rest-frame UV-to-optical spectra covering 1150-5700 \AA\ for the six AGNs were
obtained with the Hubble Space Telescope (HST). Multi-component spectral
decompositions of the HST spectra were used to measure SE emission-line widths
for the CIV, MgII, and H$\beta$ lines as well as continuum luminosities in the
spectral region around each line. We combine the new data with similar
measurements for a previous archival sample of 25 AGNs to derive the most
consistent and accurate calibrations of the CIV-based SE BH mass estimators
against the H$\beta$ reverberation-based masses, using three different measures
of broad-line width: full-width at half maximum (FWHM), line dispersion
($\sigma_{\rm line}$) and mean absolute deviation (MAD). The newly expanded
sample at redshift $z=0.005-0.234$ covers a dynamic range in BH mass
(bolometric luminosity) of $\log\ M_{\rm BH}/M_{\odot} = 6.5-9.1$ ($\log\
L_{\rm bol}/$erg s$^{\rm -1}=41.7-46.9$), and we derive the new CIV-based mass
estimators using a Bayesian linear regression analysis over this range. We
generally recommend the use of $\sigma_{\rm line}$ or MAD rather than FWHM to
obtain a less biased velocity measurement of the CIV emission line, because its
narrow-line component contribution is difficult to decompose from the
broad-line profile.
