Relighting the fire in Hickson Compact Group (HCG) 15: Magnetised fossil plasma revealed by the SKA Pathfinders and Precursors

¹ Astronomisches Institut der Ruhr-Universität Bochum (AIRUB), Universitätsstraße 150, 44801 Bochum, Germany
² Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
³ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁴ CSIRO Space & Astronomy, PO Box 1130 Bentley, WA 6102, Australia
⁵ ICRAR, The University of Western Australia, 35 Stirling Hw, 6009 Crawley, Australia
⁶ INAF – IASF Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via Alfonso Corti 12, 20133 Milano, Italy
⁷ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁸ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña MacKenna 4860, 7820436 Santiago, Chile
⁹ Naval Research Laboratory, 4555 Overlook Avenue SW, Code 7213, Washington, DC 20375, USA
¹⁰ Australian Astronomical Optics, Macquarie University, 105 Delhi Rd, North Ryde, NSW 2113, Australia
¹¹ Australia Telescope National Facility (ATNF), CSIRO, Space and Astronomy, PO Box 76 Epping, NSW 1710, Australia
¹² School of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia
¹³ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy
¹⁴ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁵ International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley, WA 6102, Australia
¹⁶ Institute of Theoretical Astrophysics, University of Oslo, PO box 1029 Blindern, Oslo 0315, Norway
¹⁷ Department of Astronomy, University of Michigan, 1085 S. University Ave., 323 West Hall, Ann Arbor, MI 48109-1107, USA
¹⁸ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
¹⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
²⁰ National Research Council Canada, Herzberg Research Centre for Astronomy and Astrophysics, Dominion Radio Astrophysical Observatory, Penticton, Canada
²¹ Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
²² ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Canberra, ACT 2611, Australia
²³ Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁴ Thüringer Landessternwarte, Sternwarte 5, D-07778 Tautenburg, Germany
²⁵ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455, USA
²⁶ South African Radio Astronomy Observatory, 2 Fir Street, Black River Park, Observatory, 7925 Cape Town, South Africa
²⁷ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune 411007, India

^⋆ Corresponding authors: riseley@astro.ruhr-uni-bochum.de, tessa.vernstrom@csiro.au

Received: 24 January 2025
Accepted: 10 March 2025

Abstract

In the context of the life cycle and evolution of active galactic nuclei (AGNs), environment plays a key role. In particular, the over-dense environments of galaxy groups, where dynamical interactions and bulk motions have significant impact, offer an excellent but under-explored window into the life cycles of AGNs and the processes that shape the evolution of relativistic plasma. Pilot survey observations with the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey have recovered diffuse emission associated with the nearby (z = 0.0228) galaxy group HCG15, which was revealed to be strongly linearly polarised. We studied the properties of this emission in unprecedented detail to settle questions about its nature and its relation to the group-member galaxies. We performed a multi-frequency spectropolarimetric study of HCG15, incorporating our ASKAP EMU observations as well as new data from MeerKAT, the LOw-Frequency ARray (LOFAR), Giant Metrewave Radio Telescope (GMRT), and Karl G. Jansky Very Large Array (VLA), along with X-ray data from XMM-Newton and optical spectra from Himalayan Chandra Telescope (HCT). Our study confirms that the diffuse structure represents remnant emission from historic AGN activity that is likely to be associated with HCG15-D, some 80 − 86 Myr ago (based on an ageing analysis). We detected significant highly linearly-polarised emission from a diffuse ‘ridge-like’ structure with a highly ordered magnetic field. Our analysis suggests that this emission is generated by the draping of magnetic field lines in the intra-group medium (IGrM). Subsequent investigations with simulations would further improve our understanding of this phenomenon. We confirm that HCG15-C is a group-member galaxy. Finally, we report the detection of thermal emission associated with a background cluster at a redshift of z ≈ 0.87 projected onto the IGrM of HCG15, which matches the position and redshift of the recent Sunyaev-Zel’dovich (SZ) detection of ACT-CL J0207.8+0209.