Wide-field LOFAR imaging of the field around the double-double radio galaxy B1834+620

A fresh view on a restarted AGN and doubeltjes

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³ Center for Astrophysical Sciences, Department of Physics & Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA
⁴ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ Department of Physics and Astronomy, University of Bologna, via Berti Pichat 6/2, 40127 Bologna, Italy
⁶ INAF–Osservatorio di Radioastronomia, via P. Gobetti, 101 40129, Bologna
⁷ INAF–Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
¹⁰ Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Department Geodesy and Remote Sensing, Telegrafenberg, 14473 Potsdam, Germany
¹¹ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹² Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹³ Shell Technology Center, Bangalore, 560099 Karnataka, India
¹⁴ SRON Netherlands Insitute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁵ CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia
¹⁶ Joint Institute for VLBI in Europe, Dwingeloo, Postbus 2, 7990 AA The Netherlands
¹⁷ University of Twente, 7522 NB Enschede, The Netherlands
¹⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁹ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
²⁰ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
²¹ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
²² Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Obs., via Cotter Road, Weston, A.C.T. 2611, Australia
²³ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
²⁴ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
²⁵ SmarterVision BV, Oostersingel 5, 9401 JX Assen, The Netherlands
²⁶ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²⁷ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁹ LPC2E – Universite d’Orleans/CNRS, 45100 Orléans Cedex 2, France
³⁰ Station de Radioastronomie de Nancay, Observatoire de Paris – CNRS/INSU, USR 704 – Univ. Orleans, OSUC, route de Souesmes, 18330 Nancay, France
³¹ Department of Physics, The George Washington University, 725 21st Street NW, Washington DC, 20052, USA
³² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
³³ Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany
³⁴ Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
³⁵ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan
³⁶ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
³⁷ Sodankylä Geophysical Observatory, University of Oulu, Tähteläntie 62, 99600 Sodankylä, Finland
³⁸ STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
³⁹ Center for Information Technology (CIT), University of Groningen, 9712 CP Groningen, The Netherlands
⁴⁰ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 av Charles André, 69561 Saint Genis Laval Cedex, France
⁴¹ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁴² Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
⁴³ Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
⁴⁴ SKA South Africa, 3rd Floor, The Park, Park Road, 7405 Pinelands, South Africa
⁴⁵ LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France

^⋆ Corresponding author: E. Orrù, e-mail: orru@astron.nl

Received: 8 May 2015
Accepted: 7 September 2015

Abstract

Context. The existence of double-double radio galaxies (DDRGs) is evidence for recurrent jet activity in active galactic nuclei (AGN), as expected from standard accretion models. A detailed study of these rare sources provides new perspectives for investigating the AGN duty cycle, AGN-galaxy feedback, and accretion mechanisms. Large catalogues of radio sources, on the other hand, provide statistical information about the evolution of the radio-loud AGN population out to high redshifts.

Aims. Using wide-field imaging with the LOFAR telescope, we study both a well-known DDRG as well as a large number of radio sources in the field of view.

Methods. We present a high resolution image of the DDRG B1834+620 obtained at 144 MHz using LOFAR commissioning data. Our image covers about 100 square degrees and contains over 1000 sources.

Results. The four components of the DDRG B1834+620 have been resolved for the first time at 144 MHz. Inner lobes were found to point towards the direction of the outer lobes, unlike standard FR II sources. Polarized emission was detected at +60 rad m^-2 in the northern outer lobe. The high spatial resolution allows the identification of a large number of small double-lobed radio sources; roughly 10% of all sources in the field are doubles with a separation smaller than 1′.

Conclusions. The spectral fit of the four components is consistent with a scenario in which the outer lobes are still active or the jets recently switched off, while emission of the inner lobes is the result of a mix-up of new and old jet activity. From the presence of the newly extended features in the inner lobes of the DDRG, we can infer that the mechanism responsible for their formation is the bow shock that is driven by the newly launched jet. We find that the density of the small doubles exceeds the density of FR II sources with similar properties at 1.4 GHz, but this difference becomes smaller for low flux densities. Finally, we show that the significant challenges of wide-field imaging (e.g., time and frequency variation of the beam, directional dependent calibration errors) can be solved using LOFAR commissioning data, thus demonstrating the potential of the full LOFAR telescope to discover millions of powerful AGN at redshift z ~ 1.