THERMAL MATERIAL IN RELATIVISTIC JETS

The properties of thermal material co-existing with non-thermal emitti ng plasma and strong magnetic fields in the powerful jets of active ga lactic nuclei (AGN) are examined. Theoretical and observational constr aints on the physical properties of this 'cold' component are determin ed. While the presence of a thermal component occupying a fraction sim ilar to 10(-8) of the jet volume is possible, it seems unlikely that s uch a component is capable of contributing significantly to the total jet energy budget, since the thermal reprocessing signatures that shou ld appear in the spectra have not, as yet, been detected.