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

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.