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Astron. Astrophys. 331, 475-484 (1998)

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Electron ageing and polarization in tailed radio galaxies

L. Feretti 1, G. Giovannini 1, 2, U. Klein 3, K.-H. Mack 1, 3, L.G. Sijbring 4 and G. Zech 3

1 Istituto di Radioastronomia del CNR, Via P. Gobetti 101, I-40129 Bologna, Italy
2 Dipartimento di Astronomia, Università di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
3 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
4 Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands

Received 16 September 1997 / Accepted 20 November 1997

Abstract

High-frequency observations of the tailed radio galaxies IC 310, NGC 1265, 3C 129, and 3C 465 have been performed with the Effelsberg 100-m telescope. For the radio galaxies IC 310, NGC 1265 and 3C 465, radio data obtained at low frequencies with the Westerbork Synthesis Radio Telescope are also available. These new radio data allow us to map the extended structure of the radio galaxies and obtain spectral and polarization information in the outermost regions.

The multi-frequency spectra were used to study the synchrotron ageing of relativistic electrons with increasing distance from the active nucleus. We found that the spectrum in each radio galaxy progressively steepens with distance, and at each location it is steeper at high frequencies. The spectra are fitted by models involving synchrotron energy losses and the critical frequency is obtained at increasing distance from the core. Assuming that the magnetic field is the equipartition value, we obtain the radiating electron lifetimes and consequently their drift velocities. Our results imply the existence of reacceleration processes or bulk motions along the tails.

The polarization data at 10.6 GHz give information on the intrinsic degree of polarized flux and the orientation of the magnetic field. We find that the polarization percentage increases along the tails, reflecting an increase of the degree of ordering of the magnetic field. The magnetic field in the tails is longitudinal.

Key words: galaxies: individual: IC 310; NGC 1265; 3C 129; 3C 465 – galaxies: magnetic fields – radio continuum: galaxies

Send offprint requests to: lferetti@ira.bo.cnr.it

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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