On the significance of relativistically hot pairs in the jets of FR II radio galaxies
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. Assuming that the relative energy contents of leptons, protons and magnetic fields are preserved across the shocks, the above implies that the large-scale jets should also be energetically dominated by leptons: $P_{\rm e,j} \gtrsim P_{\rm p,j}$. On the other hand, previous studies of small-scale jets in blazars and radio cores suggest a pair content (number of electrons and positrons per proton) of the order of $n_{\rm e}/n_{\rm p} \sim 20$. Assuming further that the particle composition of jets does not evolve beyond the blazar scales, we show that this implies an average random Lorentz factor of leptons in large-scale jets of $\barγ_{\rm e,j} \gtrsim 70(1+χ_{\rm p})(20n_{\rm p}/n_{\rm e})$, and that the protons should be mildly relativistic with $χ_{\rm p} \equiv (ε_{\rm p} + p_{\rm p})/ρ_{\rm p} c^2 \lesssim 2$, $p_{\rm p}$ the pressure of protons, $ε_{\rm p}$ the internal energy density of protons, and $ρ_{\rm p} c^2$ the rest-mass energy density of protons. We derive the necessary conditions for loading the inner jets by electron-positron pairs and proton-electron plasma, and provide arguments that heating of leptons in jets is dominated by magnetic reconnection.