Pair Plasma Dominance in pc-scale Jet of 3C345

We investigate the matter content of a pc-scale jet of 3C345. We present a general condition that a jet component becomes optically thick for synchrotron self-absorption, by extending the method originally developed by Reynolds et al. The general condition gives a lower limit of the electron number density, with the aid of the surface brightness condition. Comparing the lower limit with kinetic luminosity constraint, we can deduce its matter content. We apply the procedure to the five components of 3C345 (C2-C5 and C7) of which angular diameters and radio fluxes at the peak frequencies were obtainable from literature. Assuming an energy equipartition between the radiating particles and the magnetic field, we find that all the five components are likely dominated by a pair plasma. We discuss the tendancy that the components are gradually dominated by a normal plasma as they propagate away from the core.