VLBI, MERLIN and HST observations of the giant radio galaxy 3C 236

¹ Joint Institute for VLBI in Europe, PO Box 2, 7990 AA Dwingeloo, The Netherlands
² Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Beijing Astronomical Observatory and Astrophysics Center of the National Astronomical Observatories, CAS, 20 Datun Road, Chaoyang Beijing 100012, PR China
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Onsala Space Observatory, 439 92 Onsala, Sweden
⁶ Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
⁷ Astronomy Department, University of California, Berkeley, CA 94720-3411, USA
⁸ Department of Astronomy, University of Bologna, Via Ranzani 1, 40127 Bologna, Italy
⁹ Istituto di Radioastronomia, Via Gobetti 101, 40126 Bologna, Italy

Corresponding author: R. T. Schilizzi, schilizzi@jive.nl

Received: 9 August 2000
Accepted: 17 November 2000

Abstract

We present VLBI and MERLIN data at 1.66 and 4.99 GHz on the central component coincident with the nucleus of the giant radio galaxy, 3C 236. The nuclear radio structure is composed of two complexes of emission which are resolved on scales from 1 milli-arcsec (mas) to 1 arcsec. Oscillations with an amplitude of ~5° can be seen in the compact radio structure. Spectral index distributions are plotted at angular resolutions of 10 and 25 mas and allow us to identify the core component in the south-east emission complex. Re-examination of the HST WFPC-2 image of 3C 236 by de Koff et al. ([CITE]), shows that the normal to the dust disk in the nucleus is ~30° from the plane of the sky and within 12° of parallel to the overall orientation of the radio source. We suggest that the radio axis is also at an angle of ~30° to the plane of the sky and that the north-west jet is on the approaching side. This orientation implies an overall size of 4.5 Mpc ( km s^-1 Mpc^-1, ) for 3C 236. The coincidence of a dust feature and the south-east compact jet, within the astrometric errors, leads us to suggest that the dust may be in the form of a cloud encountered by the jet in the first ~400 pc of its journey out from the nucleus. One-sided emission at 5 GHz on 1 mas scales would suggest that the jets are ejected initially at ≤ 35° to the line of sight, but this is difficult to reconcile with the obvious orientation stability of the jet system as a whole. Free-free absorption of the counter-jet may be an alternative explanation for the one-sideness. At the resolution of WSRT data at 327 MHz, the jet to the south-east is apparently continuous over a distance of 2.5 Mpc, making this the largest jet known in the universe. It is likely, however, that activity in the nucleus of 3C 236 is episodic but with a shorter duty cycle than in the double-double sources studied by Schoenmakers et al. ([CITE]) and Kaiser et al. ([CITE]).