We revisit a method to obtain upper limits on the jet matter content combining synchrotron self-absorption constraints and the large scale bubble energy .
We use both X-ray observations , which give limits on the jet power from the energies and timescales of bubbles found in clusters of galaxies , and radio observations , which give limits on the magnetic field in the jets .
Combining the two imposes constraints on the particle number density , and hence the jet content .
Out of a sample of clusters which have clear radio bubbles , there are only two which have sufficient resolution in the radio images to give significant constraints , under the assumption that the jets are fairly steady .
The results for M87 and Perseus indicate that the radio emitting region of the jet is electron-positron dominated , assuming that the minimum of the electron energy distribution , \gamma _ { min } \sim 1 .