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

TL;DR

This study reveals a previously undetected diffuse radio remnant plasma in galaxy group NGC 507, displaced by sloshing motions, providing clear evidence of AGN feedback and its interaction with the surrounding medium.

Contribution

It presents the first detailed multi-frequency radio and X-ray analysis showing AGN remnant plasma displaced by sloshing in a galaxy group, linking radio filaments with cold fronts.

Findings

Detection of complex, filamentary diffuse radio emission in NGC 507.

Spectral ageing indicates plasma injection 240-380 Myr ago.

Spatial coincidence of radio filaments with X-ray cold fronts.

Abstract

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 144-675 MHz obtained using LOFAR and 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 derived that the plasma was first injected by the AGN 240-380 Myr ago and is now cooling. Our analysis of deep archival XMM-Newton data confirms that the system is dynamically disturbed, as previously suggested. We detect two discontinuities in the X-ray surface brightness distribution (in East and South direction) 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, which suggest that filamentary emission can be created by the cluster/group weather disrupting AGN lobes and spreading their relativistic content into the surrounding medium.