A depolarizing HI tidal tail in the western lobe of Fornax A
  Recent MeerKAT neutral hydrogen (HI) observations of Fornax A reveal tidal
material intersecting in projection the western lobe of this radio galaxy. We
found a spatial coincidence between the northern HI tail and a depolarized
structure observed for the first time with the Australian Square Kilometre
Array Pathfinder (ASKAP) at 1.2 GHz. We analyzed the properties of the rotation
measure (RM) image obtained with ASKAP data at the location of the HI tail and
in its neighborhood. We modeled the observed RM structure function to
investigate the magnetic field power spectrum at the location of the HI tail
and in a nearby control region. We found that the observed RM, in the control
region and in a region enclosing the HI tail, cannot be due to the intracluster
Faraday screen caused by the Fornax cluster. An intragroup magnetized medium
with a central magnetic field strength of 18.5 $\rm\mu$G can explain the
control region RM, but it is clear that there is an excess in correspondence
with the HI tail region. We evaluated several scenarios in which the HI tail is
either in the lobe foreground or embedded in the lobe. We determined a magnetic
field strength on the order of $\sim$9.5$-$11 $\mu$G in the HI tail, a value
consistent with constraints derived from narrowband H$\alpha$ imaging of the
ionized gas. The spatial coincidence between HI tail and depolarization
analyzed in this paper could be the first observed evidence of a magnetic field
that either has passed through a radio galaxy lobe or has survived the lobe
expansion.
