2005ApJ...619...73H

We present a new method of determining the matter content of parsec-scale jets of active galactic nuclei. By constraining the kinetic luminosity of a jet from the observed core size at a single VLBI frequency, we can infer the electron density of a radio-emitting component as a function of the composition. Comparing this density with that obtained from the theory of synchrotron self-absorption, we can determine the composition. We apply this procedure to the five components in the 3C 345 jet and find that they are likely pair plasma-dominated at 11 out of the total 21 epochs, provided that the bulk Lorentz factor is less than 15 throughout the jet. We also investigate the composition of the 3C 279 jet and demonstrate that its two components are likely pair plasma-dominated at three out of four epochs, provided that their Doppler factors are less than 10, which is consistent with observations. The conclusions do not depend on the lower cutoff energy of radiating particles.