Thermal material in relativistic jets

The properties of thermal material co-existing with non-thermal emitting plasma and strong magnetic fields in the powerful jets of active galactic nuclei (AGN) are examined. Theoretical and observational constraints on the physical properties of this 'cold' component are determined. While the presence of a thermal component occupying a fraction similar to 10(-8) of the jet volume is possible, it seems unlikely that such a component is capable of contributing significantly to the total jet energy budget, since the thermal reprocessing signatures that should appear in the spectra have not, as yet, been detected.