We provide an updated calibration of C iv \lambda 1549 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 ^ { -1 } ) .
New rest-frame UV-to-optical spectra covering 1150–5700 Å 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 C iv , Mg ii , 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 C iv-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 _ { 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 _ { BH } / M _ { \sun } = 6.5 - 9.1 ( \log L _ { 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 \sigma _ { 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 .