2001ApJ...551...72K

The mass of the central black hole in many active galactic nuclei has been estimated on the basis of the assumption that the dynamics of the broad emission line gas are dominated by the gravity of the black hole. The most commonly employed method is to estimate a characteristic size-scale r_* from reverberation mapping experiments and combine it with a characteristic velocity v_* taken from the line profiles; the inferred mass is then estimated by r_*v²_*/G. We critically discuss the evidence supporting the assumption of gravitational dynamics and find that the arguments are still inconclusive. We then explore the range of possible systematic error if the assumption of gravitational dynamics is granted. Inclination relative to a flattened system may cause a systematic underestimate of the central mass by a factor of ~(h/r)², where h/r is the aspect ratio of the flattening. The coupled effects of a broad radial emissivity distribution, an unknown angular radiation pattern of line emission, and suboptimal sampling in the reverberation experiment can cause additional systematic errors as large as a factor of 3 or more in either direction.