M 87 at metre wavelengths: the LOFAR picture

¹ Max-Planck-Institut für Astrophysik, Karl Schwarzschild Str. 1, 85741, Garching, Germany
e-mail: fdg@mpa-garching.mpg.de
² Exzellenzcluster Universe, Boltzmann Str. 2, 85748 Garching, Germany
³ Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁴ Max-Planck-Institut für Extraterrestrische Physik, Giessenbach Str., 85741 Garching, Germany
⁵ ASTRON, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
⁶ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
⁷ Leiden Observatory, Leiden University, 2300 RA, Leiden, The Netherlands
⁸ Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
⁹ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
¹⁰ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, M13 9PL, Manchester, UK
¹¹ GEPI, Observatoire de Paris-CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹² INAF – Osservatorio Astronomico di Cagliari, Strada 54, 09012 Capoterra ( CA), Italy
¹³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁴ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
¹⁵ INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
¹⁶ Jagiellonian University, ul. Orla 171, 30244 Kraków, Poland
¹⁷ School of Physics and Astronomy, University of Southampton, Highfield, SO17 1SJ, Southampton, UK
¹⁸ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
²⁰ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
²¹ Sydney Institute for Astronomy, School of Physics, The University of Sydney, Sydney, Australia
²² Harvard Smithsonian Center for Astrophysics, Garden Street 60, MA 02138, Cambridge, USA
²³ SUPA, Institute for Astronomy, Royal Observatory Edinburgh, Blackford Hill, EH9 3HJ, Edinburgh, UK
²⁴ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
²⁵ Mt Stromlo Observatory, Research School of Astronomy and Astrophysics, Australian National University, A.C.T. 2611, Weston, Australia
²⁶ Astronomisches Institut, Ruhr-Universität Bochum, 44780 Bochum, Germany
²⁷ Thüringer Landessternwarte, Sternwarte 5, 07778, Tautenburg, Germany
²⁸ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E) UMR 7328 CNRS, 45071 Orléans Cedex 02, France
²⁹ University of Oxford, Astrophysics, Denys Wilkinson Building, Keble Road, OX1 3RH, Oxford, UK
³⁰ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 av Charles André, 69561 Saint Genis Laval Cedex, France
³¹ Centre for Radio Astronomy Techniques & Technologies (RATT), Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa

Received: 11 August 2012
Accepted: 28 September 2012

Abstract

Context.M 87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4 × 10⁹   M_⊙, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo.

Aims. To place constraints on past activity cycles of the active nucleus, images of M 87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M 87 extended radio-halo.

Methods. We present the first observations made with the new Low-Frequency Array (LOFAR) of M 87 at frequencies down to 20 MHz. Three observations were conducted, at 15−30 MHz, 30−77 MHz and 116−162 MHz. We used these observations together with archival data to produce a low-frequency spectral index map and to perform a spectral analysis in the wide frequency range 30 MHz–10 GHz.

Results. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of  ≃10  μG, which increases to  ≃13  μG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of  ≃40 Myr, which in turn provides a jet kinetic power of 6−10 × 10⁴⁴  erg  s^-1.