THE LICK AGN MONITORING PROJECT: RECALIBRATING SINGLE-EPOCH VIRIAL BLACK HOLE MASS ESTIMATES
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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 <108 M . By decomposing the spectra into their AGNs and stellar components, we study the variability of the SE H line width (full width at half-maximum intensity, FWHMH or dispersion, H) and of the AGN continuum luminosity at 5100 (L 5100). From the distribution of the "virial products" ( FWHMH2 L 0.55100 or H2 L 0.55100) measured from SE spectra, we estimate the uncertainty due to the combined variability as 0.05dex (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 0.46dex (factor of 3). By comparing the H line profile of the SE, mean, and root-mean-square (rms) spectra, we find that the H line is broader in the mean (and SE) spectra than in the rms spectra by 0.1dex (25%) for our sample with FWHMH <3000kms-1. This result is at variance with larger mass BHs where the difference is typically found to be much less than 0.1dex. To correct for this systematic difference of the H line profile, we introduce a line-width dependent virial factor, resulting in a recalibration of SE BH mass estimators for low-mass AGNs. 2012. The American Astronomical Society. All rights reserved.
