The galaxy group NGC 507: Newly detected AGN remnant plasma transported by sloshing

Jets from active galactic nuclei (AGN) are known to recurrently enrich their surrounding medium with mildly relativistic particles and magnetic fields. Here, we present a detailed multi-frequency analysis of the nearby (z = 0.01646) galaxy group NGC 507. In particular, we present new high-sensitivity and high-spatial-resolution radio images in the frequency range 144-675MHz obtained using Low Frequency Array (LOFAR) and upgraded Giant Metrewave Radio Telescope (uGMRT) observations. These reveal the presence of previously undetected diffuse radio emission with complex, filamentary morphology likely related to a previous outburst of the central galaxy. Based on spectral ageing considerations, we find that the plasma was first injected by the AGN 240-380 Myr ago and is now cooling. Our analysis of deep archival X-ray Multi-Mirror Mission (XMM-Newton) data confirms that the system is dynamically disturbed, as previously suggested. We detect two discontinuities in the X-ray surface-brightness distribution (towards the east and south) tracing a spiral pattern, which we interpret as cold fronts produced by sloshing motions. The remarkable spatial coincidence observed between the newly detected arc-like radio filament and the southern concave X-ray discontinuity strongly suggests that the remnant plasma has been displaced by the sloshing motions on large scales. Overall, NGC 507 represents one of the clearest examples known to date in which a direct interaction between old AGN remnant plasma and the external medium is observed in a galaxy group. Our results are consistent with simulations that suggest that filamentary emission can be created by the cluster or group weather, disrupting AGN lobes and spreading their relativistic content into the surrounding medium.