The energetic composition of radio lobes in the FR II galaxies — estimated by comparing their radio luminosities with the powers required to inflate cavities in the external medium — seems to exclude the possibility of their energetic domination by protons .
Furthermore , if the jets were dominated by the kinetic energy of cold protons , it would be difficult to efficiently accelerate leptons in the jets ’ terminal shocks .
This suggests that the jets powering the lobes are dominated by pairs not only in number but also in enthalpy .
Such a possibility is confronted against the constraints imposed by the jet formation scenarios , and by the properties of jets on parsec and kiloparsec scales .
We find a lower limit on the pair content from the energetic dominance of leptons over protons in the radio lobes , which exceeds the estimate of n _ { e } / n _ { p } \sim 20 indicated by models of the blazar zones and of the radio cores .
This allows us to establish that the average energy of protons in the jet co-moving frame is < 2 m _ { p } c ^ { 2 } , and to estimate the number fluxes of protons and e ^ { + } e ^ { - } pairs .
The required proton flux is achievable , if the jet carries away about 1 % of the accreting matter .
Loading the jets by pairs can be achieved by pair creation using soft \gamma -ray photons from the accretion disk corona , provided that about 2-3 % of the disk radiation extends beyond 1 MeV .
Finally , we discuss possible dissipative mechanisms which may keep the jets dominated by relativistically hot pairs .