3.2 Virial Masses from Optical Variability

Surrounding the center at a distance of 0.01 to 1 pc from the black hole lies the ``broad-line} region'' (BLR). This is a compact, dense, and highly turbulent swarm of gas clouds or filaments. The clouds are illuminated by the AGN's photoionizing continuum radiation and reprocess it into emission lines that are broadened to velocities of several thousand km s^-1 by the strong gravitational field of the black hole. Then

(5)

where 1 to 3 depends on the kinematic model adopted, v is the velocity dispersion of the gas as reflected in the widths of the emission lines, and r_BLR is the radius of the BLR. The latter can be estimated by ``reverberation mapping,'' as follows. The photoionizing continuum of an AGN typically varies on timescales of days to months. In response, the emission lines vary also, but with a time delay that corresponds to the light travel time between the continuum source and the line-emitting gas. By monitoring the variations in the continuum and the emission lines in an individual object, reverberation mapping provides information on the size of the BLR. These studies also suppport the assumption that the line widths come predominantly from bound orbital motions. Applying Equation (5) suggests that Seyfert nuclei are powered by black holes with masses M_BH ~ 10⁷ to 10⁸ M, while quasar engines are more massive, with M_BH ~ 10⁸ to 10⁹ M. Since quasars also live in more massive host galaxies, this supports the emerging correlation (see the following article) between BH mass and the mass of the elliptical-galaxy-like part of the host galaxy.