THE LICK AGN MONITORING PROJECT: RECALIBRATING SINGLE-EPOCH VIRIAL BLACK HOLE MASS ESTIMATES

Abstract

We investigate the calibration and uncertainties of black hole (BH) mass estimates based on the single-epoch (SE) method, using homogeneous and high-quality multi-epoch spectra obtained by the Lick Active Galactic Nucleus (AGN) Monitoring Project for nine local Seyfert 1 galaxies with BH masses <10(8) M-circle dot. By decomposing the spectra into their AGNs and stellar components, we study the variability of the SE H beta line width (full width at half-maximum intensity, FWHMH beta or dispersion, sigma(H beta)) and of the AGN continuum luminosity at 5100 angstrom (L-5100). From the distribution of the “virial products” (proportional to FWHMH beta 2 L-5100(0.5) or sigma(2)(H beta) L-5100(0.5)) measured from SE spectra, we estimate the uncertainty due to the combined variability as similar to 0.05 dex (12%). This is subdominant with respect to the total uncertainty in SE mass estimates, which is dominated by uncertainties in the size-luminosity relation and virial coefficient, and is estimated to be similar to 0.46 dex (factor of similar to 3). By comparing the H beta line profile of the SE, mean, and root-mean-square (rms) spectra, we find that the H beta line is broader in the mean (and SE) spectra than in the rms spectra by similar to 0.1 dex (25%) for our sample with FWHMH beta < 3000 km s(-1). This result is at variance with larger mass BHs where the difference is typically found to be much less than 0.1 dex. To correct for this systematic difference of the H beta line profile, we introduce a line-width dependent virial factor, resulting in a recalibration of SE BH mass estimators for low-mass AGNs.