Calibrating Mg ii-based Black Hole Mass Estimators Using Low-to-high-luminosity Active Galactic Nuclei

Abstract

We present single-epoch black hole mass ( M BH ) estimators based on the rest-frame ultraviolet (UV) Mg ii 2798 Å and optical H β 4861 Å emission lines. To enlarge the luminosity range of active galactic nuclei (AGNs), we combine the 31 reverberation-mapped AGNs with relatively low luminosities from Bahk et al., 47 moderate-luminosity AGNs from Woo et al., and 425 high-luminosity AGNs from the Sloan Digital Sky Survey. The combined sample has a monochromatic luminosity at 5100 Å in the range erg s −1 , over the range 5.5 < 9.5. Based on the fiducial mass from the line dispersion or FWHM of H β paired with the continuum luminosity at 5100 Å, we calibrate the best-fit parameters in the black hole mass estimators using the Mg ii line. We find that the differences in the line profiles between Mg ii and H β have significant effects on calibrating the UV M BH estimators. By exploring the systematic discrepancy between the UV and optical M BH estimators as a function of AGN properties, we suggest adding a correction term in the equation for the UV mass estimator. We also find a ∼0.1 dex bias in the M BH estimation due to the difference in the spectral slope in the range 2800–5200 Å. Depending on whether the selection of M BH estimator is based on either line dispersion or FWHM and either continuum or line luminosity, the derived UV mass estimators show ≳0.1 dex intrinsic scatter with respect to the fiducial H β -based M BH .