LOFAR discovery of a 700-kpc remnant radio galaxy at low redshift

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: brienza@astron.nl
² Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
³ INAF–Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
⁴ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁵ SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁶ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul Orla 171, 30-244, Kraków, Poland
⁸ Astrophysics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK
⁹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands

Received: 15 June 2015
Accepted: 15 August 2015

Abstract

Context. Remnant radio galaxies represent the final dying phase of radio galaxy evolution in which the jets are no longer active. Remnants are rare in flux-limited samples, comprising at most a few percent. As a result of their rarity and because they are difficult to identify, this dying phase remains poorly understood and the luminosity evolution is largely unconstrained.

Aims. Here we present the discovery and detailed analysis of a large (700 kpc) remnant radio galaxy with a low surface brightness that has been identified in LOFAR images at 150 MHz.

Methods. By combining LOFAR data with new follow-up Westerbork observations and archival data at higher frequencies, we investigated the source morphology and spectral properties from 116 to 4850 MHz. By modelling the radio spectrum, we probed characteristic timescales of the radio activity.

Results. The source has a relatively smooth, diffuse, amorphous appearance together with a very weak central compact core that is associated with the host galaxy located at z = 0.051. From our ageing and morphological analysis it is clear that the nuclear engine is currently switched off or, at most, active at a very low power state. We find that the source has remained visible in the remnant phase for about 60 Myr, significantly longer than its active phase of 15 Myr, despite being located outside a cluster. The host galaxy is currently interacting with another galaxy located at a projected separation of 15 kpc and a radial velocity offset of ~ 300 km s^-1. This interaction may have played a role in the triggering and/or shut-down of the radio jets.

Conclusions. The spectral shape of this remnant radio galaxy differs from most of the previously identified remnant sources, which show steep or curved spectra at low to intermediate frequencies. Our results demonstrate that remnant radio galaxies can show a wide range of evolutionary paths and spectral properties. In light of this finding and in preparation for new-generation deep low-frequency surveys, we discuss the selection criteria to be used to select representative samples of these sources.