MERGERS, COOLING FLOWS, AND EVAPORATION

If thermal conduction is effective in the coronal plasma pervading early-type galaxies and clusters, then accretion of cold external gas and dust (mergers) can have a profound influence on the hot phase, potentially inducing symptoms hitherto attributed to a cooling flow. Among the strongest observational support for the existence of cooling flows is the presence of intermediate-temperature (approximately 10(6) K) gas with emission measure and X-ray emission-line strengths in agreement with cooling flow models. Here, quantitative solutions for the temperature structure of the X-ray gas in the conduction model are presented for up-to-date radiative cooling rates in a form that allows straightforward comparison with observation. Good agreement with the X-ray data is obtained. The relative strengths of intermediate-temperature X-ray emission lines are in significantly better agreement with a simple conduction model than with published cooling flow models. Attention is devoted to the process of evaporation in this context. The simple model for the conductive interface adopts 10(4) K as the inner boundary. Since gas clouds with temperature approximately 10(4) K are thermally stable (and highly radiative), hydrostatic solutions exist, and there does not need to be any evaporation of the infalling cloud. As well as being the physically preferred configuration (pressure equilibrium), advantages of using 10(4) K as the cloud boundary include straightforward allowance for nonlocal effects such as photoionization and the presence of other physical processes such as star formation and dust absorption internal to the clouds. The good agreement of the conduction model with optical, infrared, and X-ray data indicates that significantly more theoretical effort into this type of solution would be profitable.