Calorimetry of Active Galactic Nucleus jets: testing plasma composition in Cygnus A

TL;DR

This study investigates the plasma composition of active galactic nucleus jets in Cygnus A by analyzing cocoon dynamics and particle pressures, revealing electron/positron dominance but allowing for different pressure support scenarios.

Contribution

It introduces a method to constrain plasma composition in AGN jets by combining pressure estimates with particle density constraints, applied specifically to Cygnus A.

Findings

Electrons/positrons dominate in number density.

Either electron/positron or proton pressure support is possible.

Method effectively constrains plasma composition in AGN jets.

Abstract

We examine plasma composition of jets in active galactic nuclei through the comparison of the total pressure ($P$) with partial pressures of electrons and protons in a cocoon. The total pressure is estimated from the analysis of an expanding cocoon dynamics. We determine the average kinetic energy per particle for several representative cases of particle energy distribution such as one- and two-temperature thermal plasmas and non-thermal electrons by evaluating the dissipation of total kinetic energy of the jet into the internal energy of cocoon plasma. The number density of the total electrons/positrons ($n_{\pm}$) in the cocoon is constrained by using the particle supply from hot spots and the absence of thermal bremsstrahlung emission from radio lobes. By inserting $P$, $n_{\pm}$ and the particle energy of each population into the equation of state, the number density ($n_{p}$) and pressure ($P_{p}$) of protons in the cocoon can be constrained. Applying this method to Cygnus A, we find that (i) electron/positron ($e^{\pm}$) pairs always dominate in terms of number density, but that (ii) either an "$e^{\pm}$-supported cocoon (i.e., $P_{\pm} >P_{p}$)" or "proton-supported one (i.e, $P_{\pm} <P_{p}$)" is possible.