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Low-frequency observations of the giant radio galaxy NGC 6251

Low-frequency observations of the giant radio galaxy NGC 6251
Low-frequency observations of the giant radio galaxy NGC 6251
We present LOFAR observations at 150 MHz of the borderline FRI/FRII giant radio galaxy NGC 6251. This paper presents the most sensitive and highest resolution images of NGC 6251 at these frequencies to date, revealing for the first time a low-surface-brightness extension to the northern lobe, and a possible backflow associated with the southern lobe. The integrated spectra of components of NGC 6251 are consistent with previous measurements at higher frequencies, similar to results from other LOFAR studies of nearby radio galaxies. We find the outer structures of NGC 6251 to be either at equipartition or slightly electron dominated, similar to those of FRII sources rather than FRIs, but this conclusion remains tentative because of uncertainties associated with the geometry and the extrapolation of X-ray measurements to determine the external pressure distribution on the scale of the outer lobes. We place lower limits on the ages of the extension of the northern lobe and the backflow of the southern lobe of t ≳ 250 Myr and t ≳ 210 Myr, respectively. We present the first detection of polarization at 150 MHz in NGC 6251. Taking advantage of the high Faraday resolution of LOFAR, we place an upper limit on the magnetic field in the group of B<0.2(ΛB/10kpc)−0.5μG for a coherence scale of ΛB<60kpc and B<13μG~forΛB=240 kpc.
Galaxies: active, Polarization, Radio continuum: galaxies
0035-8711
143–159
Cantwell, T.M.
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Bray, J.D.
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Croston, J.H.
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Scaife, A.M.M.
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Mulcahy, D.D.
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Best, P.N.
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Brüggen, M.
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Brunetti, G.
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Callingham, J.R.
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Clarke, A.O.
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Hardcastle, M.J.
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Harwood, J.J.
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Heald, G.
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Heesen, V.
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Iacobelli, M.
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Jamrozy, M.
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Morganti, R.
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Orrú, E.
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O’sullivan, S.P.
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Riseley, C.J.
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Röttgering, H.J.A.
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Shulevski, A.
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Sridhar, S.S.
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Tasse, C.
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Van Eck, C.L.
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Cantwell, T.M.
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Bray, J.D.
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Croston, J.H.
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Scaife, A.M.M.
327b962f-9003-45ca-a6ea-284346c5cc85
Mulcahy, D.D.
07ba7af8-66a2-470c-93d9-a4238a2ace00
Best, P.N.
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Brüggen, M.
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Brunetti, G.
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Callingham, J.R.
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Clarke, A.O.
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Hardcastle, M.J.
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Harwood, J.J.
60420142-8884-47c6-8993-b0f8796cadb4
Heald, G.
12b841c9-c9d3-4bd7-9a69-dbf4c08294ad
Heesen, V.
b8b21826-1b25-493c-9f2a-9b0428dd1713
Iacobelli, M.
34268d77-4601-49dc-bf52-97dd1671afcd
Jamrozy, M.
f07502d3-dac4-4c67-99be-cf70dbe66d75
Morganti, R.
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Orrú, E.
d8cf2da4-cf13-416b-8ecf-f497a27e549a
O’sullivan, S.P.
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Riseley, C.J.
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Röttgering, H.J.A.
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Shulevski, A.
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Sridhar, S.S.
aa953d9f-5ab8-4419-8a03-83513d230218
Tasse, C.
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Van Eck, C.L.
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Cantwell, T.M., Bray, J.D., Croston, J.H., Scaife, A.M.M., Mulcahy, D.D., Best, P.N., Brüggen, M., Brunetti, G., Callingham, J.R., Clarke, A.O., Hardcastle, M.J., Harwood, J.J., Heald, G., Heesen, V., Iacobelli, M., Jamrozy, M., Morganti, R., Orrú, E., O’sullivan, S.P., Riseley, C.J., Röttgering, H.J.A., Shulevski, A., Sridhar, S.S., Tasse, C. and Van Eck, C.L. (2020) Low-frequency observations of the giant radio galaxy NGC 6251. Monthly Notices of the Royal Astronomical Society, 495 (1), 143–159. (doi:10.1093/mnras/staa1160).

Record type: Article

Abstract

We present LOFAR observations at 150 MHz of the borderline FRI/FRII giant radio galaxy NGC 6251. This paper presents the most sensitive and highest resolution images of NGC 6251 at these frequencies to date, revealing for the first time a low-surface-brightness extension to the northern lobe, and a possible backflow associated with the southern lobe. The integrated spectra of components of NGC 6251 are consistent with previous measurements at higher frequencies, similar to results from other LOFAR studies of nearby radio galaxies. We find the outer structures of NGC 6251 to be either at equipartition or slightly electron dominated, similar to those of FRII sources rather than FRIs, but this conclusion remains tentative because of uncertainties associated with the geometry and the extrapolation of X-ray measurements to determine the external pressure distribution on the scale of the outer lobes. We place lower limits on the ages of the extension of the northern lobe and the backflow of the southern lobe of t ≳ 250 Myr and t ≳ 210 Myr, respectively. We present the first detection of polarization at 150 MHz in NGC 6251. Taking advantage of the high Faraday resolution of LOFAR, we place an upper limit on the magnetic field in the group of B<0.2(ΛB/10kpc)−0.5μG for a coherence scale of ΛB<60kpc and B<13μG~forΛB=240 kpc.

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2004.11104 - Accepted Manuscript
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Accepted/In Press date: 21 April 2020
e-pub ahead of print date: 4 May 2020
Published date: June 2020
Keywords: Galaxies: active, Polarization, Radio continuum: galaxies

Identifiers

Local EPrints ID: 442216
URI: http://eprints.soton.ac.uk/id/eprint/442216
ISSN: 0035-8711
PURE UUID: 3d283ebc-e85c-4e3b-8132-83a91e44b4a7

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Date deposited: 09 Jul 2020 16:30
Last modified: 27 Apr 2022 10:24

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Contributors

Author: T.M. Cantwell
Author: J.D. Bray
Author: J.H. Croston
Author: A.M.M. Scaife
Author: D.D. Mulcahy
Author: P.N. Best
Author: M. Brüggen
Author: G. Brunetti
Author: J.R. Callingham
Author: A.O. Clarke
Author: M.J. Hardcastle
Author: J.J. Harwood
Author: G. Heald
Author: V. Heesen
Author: M. Iacobelli
Author: M. Jamrozy
Author: R. Morganti
Author: E. Orrú
Author: S.P. O’sullivan
Author: C.J. Riseley
Author: H.J.A. Röttgering
Author: A. Shulevski
Author: S.S. Sridhar
Author: C. Tasse
Author: C.L. Van Eck

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