A 600 kpc complex radio source at the center of Abell 3718 discovered by the EMU and POSSUM surveys^⋆

¹ INAF – Osservatorio Astronomico di Cagliari, via della Scienza 3, Selargius, Italy
e-mail: francesca.loi@inaf.it
² INAF – OAS Osservatorio di Astrofisica e Scienza delle Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, via P. Gobetti 93/2, 40129 Bologna, Italy
⁴ INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
⁵ Australia Telescope National Facility (ATNF), CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia
⁶ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455, USA
⁷ Fundación G. Galilei – INAF (Telescopio Nazionale Galileo), Rambla J. A. Fernández Pérez 7, 38712 Breña Baja (La Palma), Spain
⁸ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna (Tenerife), Spain
⁹ Departamento de Astrofísica, Univ. de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, 38205 La Laguna (Tenerife), Spain
¹⁰ Center for Astrophysics – Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹¹ School of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia
¹² School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin D09 W6Y4, Ireland
¹³ Australian Astronomical Optics, Macquarie University, 105 Delhi Rd, North Ryde, NSW 2113, Australia

Received: 7 December 2022
Accepted: 19 January 2023

Abstract

Context. Multifrequency studies of galaxy clusters are crucial for inferring their dynamical states and physics. Moreover, these studies allow us to investigate cluster-embedded sources, whose evolution is affected by the physical and dynamical condition of the cluster itself. So far, these kinds of studies have been preferentially conducted on clusters visible from the northern hemisphere due to the high-fidelity imaging capabilities of ground-based radio interferometers located there.

Aims. In this paper, we conducted a multifrequency study of the poorly known galaxy cluster Abell 3718. We investigated the unknown origin of an extended radio source with a length of ∼612 kpc at 943 MHz detected in images from the Evolutionary Map of the Universe (EMU) and POlarisation Sky Survey of the Universe’s Magnetism (POSSUM) surveys.

Methods. We analyzed optical and X-ray data to infer the dynamical state of the cluster and, in particular, the merger activity. We conducted a radio spectral index study from 943 MHz up to 9 GHz. We also evaluated the polarization properties of the brightest cluster-embedded sources to understand if they are related to the radio emission observed on larger scales.

Results. The cluster appears to be in a relaxed dynamical state, but there is clear asymmetry of the X-ray surface brightness distribution perpendicular to the direction of the largest angular extension of the radio source. The morphology of the cluster radio emission observed from 900 MHz to 9 GHz shows a system composed of a northern compact radio source and a southern radio galaxy whose jets are bent in the direction of an ultra-steep (α ≈ 3.6), thin (few tens of kpc) arc of radio emission between the first two radio sources. The spectral index gradient along the radio source and the polarization images at high frequency suggest that the thin arc is an extension of the southern radio galaxy, which may have been energized by interacting with the X-ray gas. An additional structure extending to the northwest from the southern radio galaxy may be an unusual truncated radio jet that either failed to expand or faded away due to energy losses. Deeper X-ray and radio observations are needed to better constrain the physics at play in this cluster.