FR II radio galaxies at low frequencies I: morphology, magnetic field
  strength and energetics
  Due to their steep spectra, low-frequency observations of FR II radio
galaxies potentially provide key insights in to the morphology, energetics and
spectrum of these powerful radio sources. However, limitations imposed by the
previous generation of radio interferometers at metre wavelengths has meant
that this region of parameter space remains largely unexplored. In this paper,
the first in a series examining FR IIs at low frequencies, we use LOFAR
observations between 50 and 160 MHz, along with complementary archival radio
and X-ray data, to explore the properties of two FR II sources, 3C452 and
3C223. We find that the morphology of 3C452 is that of a standard FR II rather
than of a double-double radio galaxy as had previously been suggested, with no
remnant emission being observed beyond the active lobes. We find that the
low-frequency integrated spectra of both sources are much steeper than expected
based on traditional assumptions and, using synchrotron/inverse-Compton model
fitting, show that the total energy content of the lobes is greater than
previous estimates by a factor of around 5 for 3C452 and 2 for 3C223. We go on
to discuss possible causes of these steeper than expected spectra and provide
revised estimates of the internal pressures and magnetic field strengths for
the intrinsically steep case. We find that the ratio between the equipartition
magnetic field strengths and those derived through synchrotron/inverse-Compton
model fitting remains consistent with previous findings and show that the
observed departure from equipartition may in some cases provide a solution to
the spectral versus dynamical age disparity.
