Do active galactic nucleus jets consist of a pair plasma

We investigate whether the parsec-scale jet of quasar 3C 345 is dominated by a normal (proton-electron) plasma or a pair (electron-positron) plasma. We first present a new method to compute the kinetic luminosity of a conical jet by using the core size observed at a single very long baseline interferometry frequency. The deduced kinetic luminosity gives electron densities of individual radio-emitting components as a function of the composition. We next constrain the electron density independently by using the theory of synchrotron self-absorption. Comparing the two densities, we can discriminate the composition. We then apply this procedure to the five components in the 3C 345 jet and find that they are pair-plasma dominated at 14 epochs out of the total 19 epochs, provided that the bulk Lorentz factor is less than 15 throughout the jet. The conclusion does not depend on the lower cutoff energy of radiating particles.