2017ApJ...839...93P

We provide an updated calibration of C IV λ1549 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 10^6.5–10^7.5M_☉ (10^41.7–10^43.8 erg s^–1). New rest-frame UV-to-optical spectra covering 1150-5700 Å for the six AGNs were obtained with the Hubble Space Telescope (HST). Multicomponent spectral decompositions of the HST spectra were used to measure SE emission-line widths for the C IV, Mg II, and Hβ 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 C IV-based SE BH mass estimators against the Hβ reverberation-based masses, using three different measures of broad-line width: full width at half maximum (FWHM), line dispersion (σ_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 logM_BH/M_☉=6.5–9.1 (logL_bol/ erg s^–1 = 41.7–46.9), and we derive the new C IV-based mass estimators using a Bayesian linear regression analysis over this range. We generally recommend the use of σ_line or MAD rather than FWHM to obtain a less biased velocity measurement of the C IV emission line, because its narrow-line component contribution is difficult to decompose from the broad-line profile.