Recalibrating virial black hole mass estimators

In understanding AGN physics, it is fundamental to determine black hole masses. Based on the gas kinematics of the broad-line region, black hole masses can be derived from the product of the width of the broad emission lines and the continuum/line luminosities. For a sample of 37 intermediate-luminosity AGN at z~0.4, we obtained high quality spectra (S/N~100) using the Low Resolution Imaging Spectrometer(LRIS) at the KECK telescope, in order to calibrate various black hole mass estimators based on the Mg II (2798A), the $H{\beta}$ (4861A), and the $H{\alpha}$ (6563$\bar{A}$) emission lines. Based on our multicomponent fitting analysis, we subtract continuum, FeII emission, and host galaxy starlight, reducing systematic errors in measuring emission line widths. Combining low S/N SDSS spectra with our high S/N keck spectra, we determine a set of ~30 black hole masses of the sample for each emission line. Then by comparing various sets of black hole masses, we internally calibrate each mass estimators and investigate uncertainties and limitations of each mass estimator.