FR II radio galaxies at low frequencies - I. Morphology, magnetic field strength and energetics.
Due to their steep spectra, low-frequency observations of Fanaroff-Riley type II (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 have 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 (LOw Frequency ARray) observations between 50 and 160 MHz, along with complementary archival radio and X-ray data, to explore the properties of two FR II sources, 3C 452 and 3C 223. We find that the morphology of 3C 452 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 3C 452 and 2 for 3C 223. 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.
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands.
Jeremy J
JJ
Harwood
School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK; Institute of Continuing Education, University of Cambridge, Madingley Hall, Madingley, Cambridge CB23 8AQ, UK.
Judith H
JH
Croston
Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands; National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801-0387, USA.
Huib T
HT
Intema
Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.
Adam J
AJ
Stewart
School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK.
Judith
J
Ineson
School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK.
Martin J
MJ
Hardcastle
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands.
Leith
L
Godfrey
SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK.
Philip
P
Best
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands; Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, the Netherlands.
Marisa
M
Brienza
School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK.
Volker
V
Heesen
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands.
Elizabeth K
EK
Mahony
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands; Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, the Netherlands.
Raffaella
R
Morganti
INAF, Osservatorio di Radioastronomia, Via della Scienza 5, I-09047 Selargius (Cagliari), Italy.
Matteo
M
Murgia
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands; Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University Nijmegen, PO Box 9010, NL-6500 GL Nijmegen, the Netherlands.
Emanuela
E
Orrú
Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands.
Huub
H
Röttgering
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands.
Aleksandar
A
Shulevski
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands; Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands.
Michael W
MW
Wise
X-rays: galaxies; acceleration of particles; galaxies: active; galaxies: jets; radiation mechanisms: non-thermal; radio continuum: galaxies