A reverberation-based mass for the central black hole in NGC 4151

We have undertaken a new ground-based monitoring campaign to improve the estimates of the mass of the central black hole in NGC 4151. We measure the lag time of the broad H beta line response compared to the optical continuum at 5100 angstrom and find a lag of 6.6(-0.8)(+1.1) days. We combine our data with the recent reanalysis of UV emission lines by Metzroth and coworkers to calculate a weighted mean of the black hole mass, M-BH = (4.57(-0.47)(+0.57))x10(7) M-circle dot. The absolute calibration of the black hole mass is based on normalization of the AGN black hole mass-stellar velocity dispersion (M-BH-sigma(*)) relationship to that of quiescent galaxies by Onken and coworkers. The scatter in the M-BH-sigma(*) relationship suggests that reverberation-mapping-based mass measurements are typically uncertain by a factor of 3-4.