FR-II radio galaxies at low frequencies II: spectral ageing and source
  dynamics
  In this paper, the second in a series investigating FR II radio galaxies at
low frequencies, we use LOFAR and VLA observations between 117 and 456 MHz in
addition to archival data to determine the dynamics and energetics of two radio
galaxies, 3C452 and 3C223, through fitting of spectral ageing models on small
spatial scales. We provide improved measurements for the physical extent of the
two sources, including a previously unknown low surface brightness extension to
the northern lobe of 3C223, and revised energetics based on these values. We
find spectral ages of $77.05^{+9.22}_{-8.74}$ and $84.96^{+15.02}_{-13.83}$ Myr
for 3C452 and 3C223 respectively suggesting a characteristic advance speed for
the lobes of around one per cent the speed of light. For 3C452 we show that,
even for a magnetic field strength not assumed to be in equipartition, a
disparity of factor of approximately 2 exists between the spectral age and that
determined from a dynamical standpoint. We confirm that the injection index of
both sources (as derived from the lobe emission) remains steeper than
classically assumed values even when considered on well resolved scales at low
frequencies, but find an unexpected sharp discontinuity between the spectrum of
the hotspots and the surrounding lobe emission. We suggest that this
discrepancy is due to the absorption of hotspot emission and/or non-homogeneous
and additional acceleration mechanisms and, as such, hotspots should not be
used in the determination of the underlying initial electron energy
distribution.
