LOFAR MSSS: Discovery of a 2.56 Mpc giant radio galaxy associated with a disturbed galaxy group

¹ University of Manchester, Jodrell Bank Centre for Astrophysics, Manchester, M139PL, UK
e-mail: alex.clarke-3@manchester.ac.uk
² CSIRO Astronomy and Space Science, 26 Dick Perry Avenue, Kensington WA 6151, Australia
³ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
⁵ Astronomy Dep., University of Cape Town, 7701 Rondebosch, RSA
⁶ Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, AL10 9AB, UK
⁷ Hamburger Sternwarte – Hamburg University, Gojenbergsweg 112, 21029 Hamburg, Germany
⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098XH, Amsterdam, The Netherlands
⁹ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
¹⁰ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹¹ Hamburger Sternwarte, Hamburg University, Gojenbergsweg 112, 21029 Hamburg, Germany
¹² Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany
¹³ Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA
¹⁴ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
¹⁵ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Krakow, Poland
¹⁶ Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹⁷ INAF–Istituto di Radioastronomia, via P. Gobetti, 101, 40129 Bologna, Italy
¹⁸ Univ. Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 9 avenue Charles André, 69230 Saint-Genis-Laval, France
¹⁹ Mullard Space Science Laboratory (MSSL), University College London (UCL), Holmbury St Mary, Surrey RH5 6NT, UK
²⁰ School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand
²¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²² Institute for Astronomy (IfA), University of Edinburgh, Royal Observatory, Blackford Hill, EH9 3 HJ Edinburgh, UK
²³ Astronomisches Institut der Universitat Bochum, Universitatsstr. 150, 44801 Bochum, Germany
²⁴ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
²⁵ Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
²⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 28 November 2016
Accepted: 27 January 2017

Abstract

We report on the discovery in the LOFAR Multifrequency Snapshot Sky Survey (MSSS) of a giant radio galaxy (GRG) with a projected size of 2.56 ± 0.07 Mpc projected on the sky. It is associated with the galaxy triplet UGC 9555, within which one is identified as a broad-line galaxy in the Sloan Digital Sky Survey (SDSS) at a redshift of 0.05453 ± 1 × 10^-5, and with a velocity dispersion of 215.86 ± 6.34 km s^-1. From archival radio observations we see that this galaxy hosts a compact flat-spectrum radio source, and we conclude that it is the active galactic nucleus (AGN) responsiblefor generating the radio lobes. The radio luminosity distribution of the jets, and the broad-line classification of the host AGN, indicate this GRG is orientated well out of the plane of the sky, making its physical size one of the largest known for any GRG. Analysis of the infrared data suggests that the host is a lenticular type galaxy with a large stellar mass (log  M/M_⊙ = 11.56 ± 0.12), and a moderate star formation rate (1.2 ± 0.3 M_⊙/ year). Spatially smoothing the SDSS images shows the system around UGC 9555 to be significantly disturbed, with a prominent extension to the south-east. Overall, the evidence suggests this host galaxy has undergone one or more recent moderate merger events and is also experiencing tidal interactions with surrounding galaxies, which have caused the star formation and provided the supply of gas to trigger and fuel the Mpc-scale radio lobes.