Low-frequency observations of the Giant Radio Galaxy NGC 6251

Author:

Cantwell T M1ORCID,Bray J D1ORCID,Croston J H2,Scaife A M M1,Mulcahy D D1,Best P N3,Brüggen M4,Brunetti G5,Callingham J R6ORCID,Clarke A O1,Hardcastle M J7ORCID,Harwood J J7ORCID,Heald G8ORCID,Heesen V49,Iacobelli M6,Jamrozy M10,Morganti R611ORCID,Orrú E6,O’Sullivan S P4,Riseley C J8512,Röttgering H J A13,Shulevski A14ORCID,Sridhar S S6,Tasse C1516,Van Eck C L17

Affiliation:

1. JBCA, Dept. of Physics & Astronomy, University of Manchester, Manchester M13 9PL, UK

2. School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

3. SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK

4. Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

5. INAF — Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy

6. ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, the Netherlands

7. Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, Hertfordshire AL10 9AB, UK

8. CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia

9. School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

10. Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30–244 Krakow, Poland

11. Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, the Netherlands

12. Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, via P. Gobetti 93/2, 40129 Bologna, Italy

13. Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands

14. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, the Netherlands

15. GEPI, Observatoire de Paris, CNRS, Universite Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France

16. Dept. of Physics & Electronics, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa

17. Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada

Abstract

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 polarisation 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/10 kpc)−0.5 μG for a coherence scale of ΛB < 60 kpc and B < 13 μG for ΛB = 240 kpc.

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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