The peculiar radio galaxy 4C 35.06: a case for recurrent AGN activity?

¹ University of Groningen, Kapteyn Astronomical Institute, Landleven 12, 9747 AD Groningen, The Netherlands
e-mail: pdb@astro.rug.nl
² ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: shulevski@astron.nl; morganti@astron.nl
³ INAF–Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (Cagliari), Italy
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Department of Physics and Astronomy, The Open University, Milton Keynes MK7 6AA, UK
⁶ RAL Space, The Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
⁷ Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁸ Toruń Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziacka 5, 87-100 Toruń, Poland
⁹ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul Orla 171, 30-244 Kraków, Poland
¹⁰ SUPA, Institute for Astronomy, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹¹ LeidenObservatory, Leiden University, PO Box 9513, 2333 RA Leiden, The Netherlands
¹² Helmholtz-Zentrum Potsdam, DeutschesGeoForschungsZentrum GFZ, Department 1: Geodesy and Remote Sensing, Telegrafenberg, A17, 14473 Potsdam, Germany
¹³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁴ CSIRO Australia Telescope National Facility, PO Box 76, NSW 1710 Epping, Australia
¹⁵ University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands
¹⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁷ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁸ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
¹⁹ Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Obs., via Cotter Road, ACT 2611 Weston, Australia
²⁰ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
²¹ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
²² SmarterVision BV, Oostersingel 5, 9401 JX Assen, The Netherlands
²³ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²⁴ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²⁵ LPC2E–Université d’Orléans/CNRS, 45071 Orléans Cedex 2, France
²⁶ Station de Radioastronomie de Nancay, Observatoire de Paris – CNRS/INSU, USR 704 – Univ. Orléans, OSUC, route de Souesmes, 18330 Nançay, France
²⁷ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
²⁸ Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
²⁹ Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
³⁰ Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
³¹ National Astronomical Observatory of Japan, 2 Chome-21-1 Osawa, Mitaka, Tokyo, Japan
³² Sodankylä Geophysical Observatory, University of Oulu, Tähteläntie 62, 99600 Sodankylä, Finland
³³ STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
³⁴ University of Groningen, Center for Information Technology (CIT), Nettelbosje 1, 9747 AJ Groningen, The Netherlands
³⁵ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 Saint-Genis Laval Cedex, France
³⁶ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
³⁷ Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
³⁸ SKA South Africa, 3rd Floor, The Park, Park Road, 7405 Pinelands, South Africa
³⁹ LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France
⁴⁰ IRA–INAF, via P. Gobetti 101, 40129 Bologna, Italy
⁴¹ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France

Received: 26 November 2014
Accepted: 16 April 2015

Abstract

Using observations obtained with the LOw Fequency ARray (LOFAR), the Westerbork Synthesis Radio Telescope (WSRT) and archival Very Large Array (VLA) data, we have traced the radio emission to large scales in the complex source 4C 35.06 located in the core of the galaxy cluster Abell 407. At higher spatial resolution (~ 4″), the source was known to have two inner radio lobes spanning 31 kpc and a diffuse, low-brightness extension running parallel to them, offset by about 11 kpc (in projection). At 62 MHz, we detect the radio emission of this structure extending out to 210 kpc. At 1.4 GHz and intermediate spatial resolution (~ 30″), the structure appears to have a helical morphology. We have derived the characteristics of the radio spectral index across the source. We show that the source morphology is most likely the result of at least two episodes of AGN activity separated by a dormant period of around 35 Myr. The outermost regions of radio emission have a steep spectral index (α< − 1), indicative of old plasma. We connect the spectral index properties of the resolved source structure with the integrated fluxdensity spectral index of 4C 35.06 and suggest an explanation for its unusual integrated flux density spectral shape (a moderately steep power law with no discernible spectral break), possibly providing a proxy for future studies of more distant radio sources through inferring their detailed spectral index properties and activity history from their integrated spectral indices. The AGN is hosted by one of the galaxies located in the cluster core of Abell 407. We propose that it is intermittently active as it moves in the dense environment in the cluster core. In this scenario, the AGN turned on sometime in the past, and has produced the helical pattern of emission, possibly a sign of jet precession/merger during that episode of activity. Using LOFAR, we can trace the relic plasma from that episode of activity out to greater distances from the core than ever before. Using the the WSRT, we detect H I in absorption against the center of the radio source. The absorption profile is relatively broad (FWHM of 288 kms^-1), similar to what is found in other clusters. The derived column density is N_HI ~ 4 × 10²⁰ cm^-2 for a T_spin = 100 K. This detection supports the connection – already suggested for other restarted radio sources – between the presence of cold gas and restarting activity. The cold gas appears to be dominated by a blue-shifted component although the broad H I profile could also include gas with different kinematics. Understanding the duty cycle of the radio emission as well as the triggering mechanism for starting (or restarting) the radio-loud activity can provide important constraints to quantify the impact of AGN feedback on galaxy evolution. The study of these mechanisms at low frequencies using morphological and spectral information promises to bring new important insights in this field.