During the 1950s and 1960s, the imaging of extragalactic radio sources steadily improved, and most resolved sources appeared to be simple double sources surrounding either an optical galaxy or quasar. This led to theories involving explosions in the parent object which ejected clouds of magnetized plasma and relativistic particles (e.g., De Young and Axford 1967). It soon became clear however, that in the simple versions of these models, radiation and adiabatic expansion losses were too large (e.g., Scheuer 1974). Thus, some sort of continuous supply of energy and, in some cases, in situ particle acceleration were required, and an external medium surrounding the radio-emitting region appeared to be necessary to keep the source from too dispersing rapidly.
Models involving ejection of multiple blobs of plasma (or plasmons) were suggested by Christiansen (1973) and others to overcome the rapid losses. A second class of model, supported by Saslaw et al. (1974) involved the ejection of supermassive objects, via three-body interactions in the nucleus, which then supplied relativistic particles continuously to maintain the source. A third class of models, first proposed by Rees (1971) and colleagues, involved resupply of the radio lobes by a continuous beam of particles or waves.