Calibrating Mg II-based Black Hole Mass Estimators with Hβ Reverberation Measurements

Abstract

We present the new calibration of black hole mass ({M}_BH}) estimators based on the Mg II λ2798 broad emission line using the Hβ reverberation-mapped (RM) sample of 31 active galactic nuclei (AGNs) that have high-quality UV spectra. The updated and enlarged sample at redshift z < 0.16 covers a large dynamic range in {M}_BH} (i.e., {10}^5.7{--}{10}^9.0 {M}_⊙ ) and bolometric luminosity (i.e., {10}^42.5{--}{10}^46.8 erg s^-1). We also investigate the mass estimators by adding a sample from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project, which consists of 24 AGNs at a higher redshift range, z = 0.36-0.89. We determine the line width of the Mg II line using FWHM or line dispersion (σ _line), which are respectively used for the {M}_BH} recipe. Comparing with the Hβ reverberation masses, we provide several calibrations for the Mg II-based mass, the best of which is consistent with the virial assumption and photoionization expectation (i.e., {M}_BH}\propto {L}₃₀₀₀^0.5 {FWHM}}_Mg {{II}}}² and {M}_BH}\propto {L}₃₀₀₀^0.5 {σ }_Mg {{II}}}²). The derived {M}_BH} based the Mg II estimator is consistent with the Hβ reverberation mass within a factor of 2.