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FR II radio galaxies at low frequencies – I. morphology, magnetic field strength and energetics

FR II radio galaxies at low frequencies – I. morphology, magnetic field strength and energetics
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.
0035-8711
4443-4455
Harwood, Ian
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Croston, Judith
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Intema, Huib T.
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Stewart, Adam J.
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Ineson, Judith
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Hardcastle, Martin J.
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Godfrey, Leith
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Best, Philip
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Brienza, Marisa
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Heesen, Volker
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Mahony, Elizabeth K.
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Morganti, Raffaella
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Murgia, Matteo
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Orrú, Emanuela
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Röttgering, Huub
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Shulevski, Aleksandar
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Wise, Michael W.
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Harwood, Ian
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Croston, Judith
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Intema, Huib T.
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Stewart, Adam J.
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Ineson, Judith
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Hardcastle, Martin J.
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Godfrey, Leith
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Best, Philip
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Brienza, Marisa
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Heesen, Volker
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Mahony, Elizabeth K.
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Morganti, Raffaella
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Murgia, Matteo
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Orrú, Emanuela
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Röttgering, Huub
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Shulevski, Aleksandar
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Wise, Michael W.
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Harwood, Ian, Croston, Judith and Intema, Huib T. et al. (2016) FR II radio galaxies at low frequencies – I. morphology, magnetic field strength and energetics. Monthly Notices of the Royal Astronomical Society, 458 (4), 4443-4455. (doi:10.1093/mnras/stw638).

Record type: Article

Abstract

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.

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Accepted/In Press date: 11 March 2016
e-pub ahead of print date: 17 March 2016
Published date: 1 June 2016
Organisations: Astronomy Group

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Local EPrints ID: 401967
URI: http://eprints.soton.ac.uk/id/eprint/401967
ISSN: 0035-8711
PURE UUID: 7d09018a-f1f0-4b3a-8505-f704699cf305

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Date deposited: 26 Oct 2016 10:43
Last modified: 16 Dec 2019 19:39

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Contributors

Author: Ian Harwood
Author: Judith Croston
Author: Huib T. Intema
Author: Adam J. Stewart
Author: Judith Ineson
Author: Martin J. Hardcastle
Author: Leith Godfrey
Author: Philip Best
Author: Marisa Brienza
Author: Volker Heesen
Author: Elizabeth K. Mahony
Author: Raffaella Morganti
Author: Matteo Murgia
Author: Emanuela Orrú
Author: Huub Röttgering
Author: Aleksandar Shulevski
Author: Michael W. Wise

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