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 144-675 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.
