In the context of the life cycle and evolution of active galactic nuclei (AGN), the environment plays an important 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 AGN 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 recovered diffuse emission associated with the nearby (z = 0.0228) galaxy group HCG15, which was revealed to be strongly linearly polarised. Riseley et al. study the properties of this emission in unprecedented detail to settle open questions about its nature and its relation to the group-member galaxies. They perform a multi-frequency spectropolarimetric study of HCG15 incorporating our ASKAP EMU observations as well as new data from MeerKAT, LOFAR, the GMRT, and the Karl G. Jansky Very Large Array (VLA), plus X-ray data from XMM-Newton and optical spectra from the Himalayan Chandra Telescope (HCT). The study confirms that the diffuse structure represents remnant emission from historic AGN activity, likely associated with HCG15-D, some 80-86 Myr ago (based on ageing analysis). Significant highly linearly-polarised emission is detected from a diffuse ‘ridge’-like structure with a highly ordered magnetic field. The analysis suggests that this emission is generated by draping of magnetic field lines in the intra-group medium (IGrM), although further exploration with simulations would aid our understanding. Riseley et al. confirm that HCG15-C is a group-member galaxy. Finally, they report the detection of thermal emission associated with a background cluster at redshift z ~ 0.87 projected onto the IGrM of HCG15, which matches the position and redshift of the recent SZ detection of ACT-CL J0207.8+0209.

The image presents Colour-composite images of HCG15. The optical RGB image is constructed using i-, r- and g-bands from Data Release 2 of the Dark Energy Survey (DES). Green colour and contours denote their XMM-Newton X-ray data in the 0.7−2.0 keV range, smoothed with a Gaussian of 7 pixels (14 arcsec) FWHM. Radio data from ASKAP at 943MHz (lower-left) and MeerKAT at 2.4GHz (lower-right) are shown, with red colour and contours denoting continuum emission and blue colour denoting linearly-polarised emission. All radio data is shown at 20 arcsec resolution, as indicated by the hatched circle in the lower-left corner. Contours start at 3σ and scale by a factor of √2. Cyan vectors denote the orientation of the magnetic field, with vector size proportional to the fractional polarisation. Yellow circles and labels identify the six group-member galaxies; cyan ‘+’ sign in the top-right panel denotes the SZ position of a background cluster at z≃0.88 reported by Klein et al. (2024), with the circle tracing a 500 kpc radius at that redshift.