Evidence for a significant mixture of electron/positron pairs in FRII jets constrained by cocoon dynamics

TL;DR

This study analyzes the plasma composition of FRII radio galaxy jets, finding that a significant positron component is necessary, regardless of jet power or cocoon size, based on cocoon pressure dynamics.

Contribution

It provides the first direct evidence that positrons are a necessary component in the plasma of FRII jets, constraining their number density relative to protons.

Findings

Positron-free plasma models are ruled out for all four galaxies.

The ratio of electrons/positrons to protons exceeds two.

Cocoon support mechanisms depend on thermal electron/positron presence.

Abstract

We examine the plasma composition of relativistic jets in four FRII radio galaxies by analyzing the total cocoon pressure in terms of partial pressures of thermal and non-thermal electrons/positrons and protons. The total cocoon pressure is determined by cocoon dynamics via comparison of theoretical model with the observed cocoon shape. By inserting the observed number density of non-thermal electrons/positrons and the upper limit of thermal electron/positron number density into the equation of state, the number density of protons is constrained. We apply this method to four FRII radio galaxies (Cygnus A, 3C 219, 3C 223 and 3C 284), for which the total cocoon pressures have been already evaluated. We find that the positron-free plasma comprising of protons and electrons is ruled out, when we consider plausible particle distribution functions. In other words, the mixture of positrons is required for all four FRII radio galaxies; the number density ratio of electrons/positrons to protons is larger than two. Thus, we find that the plasma composition is independent of the jet power and the size of cocoons. We also investigate the additional contribution of thermal electrons/positrons and protons on the cocoon dynamics. When thermal electrons/positrons are absent, the cocoon is supported by the electron/ proton plasma pressure, while both electron/positron pressure supported and electron/proton plasma pressure supported cocoons are allowed if the number density of thermal electrons/positrons is about 10 times larger than that of non-thermal ones.