A multifrequency study of giant radio sources III. Dynamical age vs.
  spectral age of the lobes of selected sources
  The dynamical ages of the opposite lobes of selected giant radio sources are
estimated using the DYNAGE algorithm of Machalski et al., and compared with
their spectral ages estimated and studied by Jamrozy et al. in Paper II. As
expected, the DYNAGE fits give slightly different dynamical ages and other
model's parameters for the opposite lobes modelled independently each other,
e.g. the age ratios are found between ~1.1 to ~1.4. Demanding similar values of
the jet power and the radio core density for the same source, we look for a
self-consistent solution for the opposite lobes, which results in different
density profiles along them found by the fit. We also show that a departure
from the equipartition conditions assumed in the model, justified by X-ray
observations of the lobes of some nearby radio galaxies, and a relevant
variation of the magnetic-field strengths may provide an equalisation of the
lobes' ages. A comparison of the dynamical and spectral ages shows that a ratio
of the dynamical age to the spectral age of the lobes of investigated giant
radio galaxies is between ~1 and ~5, i.e. is similar to that found for smaller
radio galaxies (e.g. Parma et al. 1999). Supplementing possible causes for this
effect already discussed in the literature, like uncertainty of assumed
parameters of the model, an influence of a possible departure from the energy
equipartition assumption, etc. Arguments are given to suggest that DYNAGE can
better take account of radiative effects at lower frequencies than the
spectral-ageing analysis.The DYNAGE algorithm is especially effective for
sources at high redshifts, for which an intrinsic spectral curvature is shifted
to low frequencies.
