Stability of cloud orbits in the broad-line region of active galactic nuclei

We investigate the global dynamic stability of spherical clouds in the broad-line region (BLR) of active galactic nuclei (AGN), exposed to radial radiation pressure, gravity of the central black hole (BH) and centrifugal forces assuming the clouds adapt their size according to the local pressure. We consider both isotropic and anisotropic light sources. In both cases, stable orbits exist also for very sub-Keplerian rotation for which the radiation pressure contributes substantially to the force budget. We demonstrate that highly eccentric, very sub-Keplerian stable orbits may be found. This gives further support for the model of Marconi et al., who pointed out that BH masses might be significantly underestimated if radiation pressure is neglected. That model improved the agreement between BH masses derived in certain active galaxies based on BLR dynamics, and BH masses derived by other means in other galaxies by inclusion of a luminosity-dependent term. For anisotropic illumination, energy is conserved for averages over long time intervals only, but not for individual orbits. This leads to Rosetta orbits that are systematically less extended in the direction of maximum radiation force. Initially isotropic relatively low column density systems would therefore turn into a disc when an anisotropic AGN is switched on.