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

Abstract

We present single-epoch black hole mass (MBH) estimators based on the rest-frame ultraviolet (UV) Mg II 2798 angstrom and optical H beta 4861 angstrom 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 moderateluminosity 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 A in the range log lambda L-5100 similar to 41.3-46.5 erg s(-1), over the range 5.5 < log(M-BH/M-circle dot)<9.5. Based on the fiducial mass from the line dispersion or FWHM of H beta paired with the continuum luminosity at 5100 A, 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 beta 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 Delta M = -1.44 log(FWHMMgII/sigma(MgII)) + 0.33 in the equation for the UV mass estimator. We also find a similar to 0.1 dex bias in the MBH estimation due to the difference in the spectral slope in the range 2800-5200 angstrom. 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 greater than or similar to 0.1 dex intrinsic scatter with respect to the fiducial H beta-based M-BH.