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Astron. Astrophys. 356, 63-72 (2000)


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The giant radio galaxy 8C 0821+695 and its environment

L. Lara 1, K.-H. Mack 2,3, M. Lacy 4, U. Klein 3, W.D. Cotton 5,6, L. Feretti 2, G. Giovannini 2,7 and M. Murgia 2,8

1 Instituto de Astrofísica de Andalucía (CSIC), Apdo. 3004, 18080 Granada, Spain
2 Istituto di Radioastronomia (CNR), Via P. Gobetti 101, 40129 Bologna, Italy
3 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
4 IGPP, L-413, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
5 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
6 Sterrewacht Leiden, Niels Bohrweg 2, 2300-RA Leiden, The Netherlands
7 Università di Bologna, Dipartimento di Fisica, Via B. Pichat 6/2, 40127 Bologna, Italy
8 Università di Bologna, Dipartimento di Astronomia, Via Ranzani 2, 40127 Bologna, Italy

Received 3 November 1999 / Accepted 31 January 2000

Abstract

We present new VLA and Effelsberg observations of the radio galaxy 8C 0821+695. We have obtained detailed images in total intensity and polarization of this 2 Mpc sized giant. The magnetic field has a configuration predominantly parallel to the source main axis. We observe Faraday rotation at low frequencies, most probably produced by an ionized medium external to the radio source. The spectral index distribution is that typical of FR II radio galaxies, with spectral indices gradually steepening from the source extremes towards the core. Modeling the spectrum in the lobes using standard synchrotron loss models yields the spectral age of the source and the mean velocity of the jet-head with respect to the lobe material. The existence of a possible backflow in the lobe is considered to relate spectral with dynamical determinations of the age and the velocity with respect to the external medium. Through a very simple model, we obtain a physical characterization of the jets and the external medium in which the radio galaxy expands. The results in 8C 0821+695 are consistent with a relativistic jet nourishing the lobes which expand in a hot, low density halo. We infer a deceleration of the source expansion velocity which we explain through a progressive increase in the hot-spot size.

Key words: galaxies: individual: 8C 0821+695 – galaxies: active – galaxies: cooling flows – radio continuum: galaxies

Send offprint requests to: L. Lara

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000

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