Boundary Layers in Extragalactic Jets
The radio morphology of extragalactic radio sources, in particular the FRI objects, has long been recognized to be suggestive of an outflow which is undergoing a strong interaction with the surrounding medium. Two essential elements must be noted in considering this interaction. First, it is inevitable. Second, the interaction will always provide mass and energy transfer from the jet to the ambient medium, and thus it is a loss term in the overall energy balance of the radio source. As will be seen, these losses can be comparable to the overall radio luminosity and hence are a nontrivial constraint on the energy requirements placed on the central engine.