Spectral ageing analysis and dynamical analysis of the double-double
  radio galaxy J1548-3216
  Using the new low-frequency and high-frequency radio images of this galaxy,
we determined the shape of the spectrum along its lobes and performed the
classical spectral-ageing analysis. On the other hand, we applied the
analytical model of the jet's dynamics, which allowed us to derive the physical
conditions for the source's evolution during the original jet propagation
through the unperturbed IGM, as well as those when the restarted new jet
propagates inside the outer cocoon formed by the old jet material that passed
through the jet terminal shock. The dynamical age estimate of the outer and the
inner lobes is 132+/-28 Myr and ~9+/-4 Myr, respectively. The synchrotron age
in the outer lobes systematically rises from ~25 Myr in the vicinity of the
lobes' edges to about 65-75 Myr in the centre of the old cocoon. These ages
imply an average expansion speed along the jets' axis: (0.012+/-0.003)c in the
outer lobes and (0.058+/-0.025)c in the inner lobes, but the latter speed would
be ~0.25c when they were of age less than 1 Myr. We find that the jet power
during the restarted activity is about ten-fold fainter than that of the
original jet. Similar disproportion is found for the internal pressures and the
magnetic field strengths in the old cocoon and those in the inner lobes. This
disproportion can be effectively reduced by assuming the same equations of
state for the emitting particles and the magnetic fields within the old and the
new lobes. However, we think that our assumption of the non-relativistic
equation of state for the old cocoon and the relativistic one for the new lobes
is more justified.
