BOUNCING CLOUDS - A MODEL FOR THE QUASAR BROAD-LINE REGION

Clouds held in pressure equilibrium by a high-pressure confining medium in quasar atmospheres can be attracted toward an equilibrium radius provided the confining pressure decreases sufficiently rapidly with distance from the central massive object. At the equilibrium radius gravity is exactly balanced by radiation forces. Oscillations of dense, fully ionized clouds normal to the equilibrium radius create emission-line profiles very similar to those observed. The profile is independent of the masses of the clouds or their shapes. In such a model the red and blue sides of emission lines should respond to changes in the quasar luminosity (or pressure in the broad-line region) approximately simultaneously, as observed. Radial cloud motions do not interfere with the possible existence of an accretion disk; the line profile is unchanged if the disk is opaque to line radiation and is invariant to the angle between the line of sight and the symmetry axis of the visible hemisphere above the disk. Masses obtained by applying the widths of emission lines to the virial theorem are underestimated by about approximately 10-20; the significantly greater masses implied by the bouncing cloud model very strongly enhance the likelihood of massive black holes in active galaxies and quasars. It is likely that broad lines observed from ions of low excitation can also be explained by an extension of the bouncing cloud model. Electron scattering of line radiation from ambient plasma may account for the blueshifts and profile asymmetries observed in broad lines of high excitation.