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. 2008, who pointed out that
black hole masses might be significantly underestimated if radiation pressure
is neglected. That model improved the agreement between black hole masses
derived in certain active galaxies based on BLR dynamics, and black hole 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 disk when an anisotropic AGN is switched on.
