The origin of broad emission line clouds in active galactic nuclei

Numerous arguments suggest that the broad emission lines characteristic of many types of active galactic nuclei are formed in comparatively cool clouds embedded in a much hotter medium of roughly equal pressure. Here it is shown that a cycle exists in which cooler condensations form within the hot medium via a finite-amplitude thermal instability, drop to temperatures characteristic of photoionization equilibrium, are destroyed by hydrodynamic processes, and then return their material to the hot medium. Clouds of characteristic pressure about 10 to the 14th K/cu cm and thickness about 10 to the 23rd/sq cm are produced by this mechanism, in agreement with the pressure and thickness inferred from photoionization model analysis of observed emission-line ratios. Only a fraction of the hot medium's mass must encounter a finite-amplitude perturbation in order to produce the observed mass in clouds.