THE STABILITY OF QSO AGN BROAD EMISSION-LINE CLOUDS

This paper presents the results of a numerical linear stability analysis of QSO/AGN emission-line clouds (ELCs) embedded within a confining hot intercloud medium (HIM). To be as realistic as possible, this analysis is performed on a self-consistent ELC/HIM system in radiative, pressure, and thermal equilibrium. Energy transport includes the effects of an external broken power law, multielement optically thick line radiation, thermal conduction of heat from the HIM, thermal bremsstrahlung, and Compton processes. Momentum balance includes both gas and radiation pressure. Several conclusions can be drawn from our equilibrium ELC/HIM solution. For example, conduction is found to be unimportant to the total energy budget, but is important to the ELC/HIM interface dynamics. An important result is that radiation pressure exceeds the gas pressure in certain regions of our equilibrium atmosphere, and is extremely important to ELC structure. Two major instabilities, one thermal and one dynamic, were discovered. While the bulk of the ELC is thermally stable, the transition region between the ELC and HIM was found to be thermally unstable with instability growth times of approximately 10(3) s. This results in evaporation of the ELC in approximately 10 yr. In addition, the high-ionization zone (HIZ) of the ELC (i.e., the Ly-alpha-forming region) is dynamically unstable to the growth of radiatively driven sound waves, resulting in a mass outflow from the HIZ. Both the external radiation field and the force of trapped line radiation contribute to this instability. The growth time for this instability due to thc external radiation field alone (approximately 3 x 10(6) s) is smaller than first estimated by Mathews. The effect of the trapped line radiation is more important, however, being both larger in magnitude and more locally concentrated. This force causes sound waves to grow significantly in approximately 10(6) s, approximately the time for the waves to cross the HIZ. These instabilities suggest that if the ELCs are accurately described by the standard model, they are in a constant state of flux, with many births and deaths occurring over the time required for an ELC to cross the broad-line region.