Spectral ageing in the lobes of cluster-centre FR-II radio galaxies
  Recent investigations have shown that many parameters and assumptions made in
the application of spectral ageing models to FR-II radio galaxies (e.g.
injection index, uniform magnetic field, non-negligible cross-lobe age
variations) may not be as reliable as previously thought. In this paper we use
new VLA observations, which allow spectral curvature at GHz frequencies to be
determined in much greater detail than has previously been possible, to
investigate two cluster-centre radio galaxies, 3C438 and 3C28. We find that for
both sources the injection index is much steeper than the values traditionally
assumed, consistent with our previous findings. We suggest that the Tribble
model of spectral ageing provides the most convincing description when both
goodness-of-fit and physically plausibility are considered, but show that even
with greatly improved coverage at GHz frequencies, a disparity exists in
cluster-centre FR-IIs when spectral ages are compared to those determined from
a dynamical viewpoint. We find for 3C438 that although the observations
indicate the lobes are expanding, its energetics suggest that the radiating
particles and magnetic field at equipartition cannot provide the necessary
pressure to support the lobes, similar to other cluster-centre source such as
Cygnus A. We confirm that small scale, cross-lobe age variations are likely to
be common in FR-II sources and should be properly accounted for when
undertaking spectral ageing studies. Contrary to the assumption of some
previous studies, we also show that 3C28 is an FR-II (rather than FR-I) source,
and suggest that it is most likely a relic system with the central engine being
turned off between 6 and 9 Myrs ago.
