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Recent MeerKAT neutral hydrogen (HI) observations of Fornax A reveal tidalmaterial intersecting in projection the western lobe of this radio galaxy. Wefound a spatial coincidence between the northern HI tail and a depolarizedstructure observed for the first time with the Australian Square KilometreArray Pathfinder (ASKAP) at 1.2 GHz. We analyzed the properties of the rotationmeasure (RM) image obtained with ASKAP data at the location of the HI tail andin its neighborhood. We modeled the observed RM structure function toinvestigate the magnetic field power spectrum at the location of the HI tailand in a nearby control region. We found that the observed RM, in the controlregion and in a region enclosing the HI tail, cannot be due to the intraclusterFaraday screen caused by the Fornax cluster. An intragroup magnetized mediumwith a central magnetic field strength of 18.5 $\rm\mu$G can explain thecontrol region RM, but it is clear that there is an excess in correspondencewith the HI tail region. We evaluated several scenarios in which the HI tail iseither in the lobe foreground or embedded in the lobe. We determined a magneticfield strength on the order of $\sim$9.5$-$11 $\mu$G in the HI tail, a valueconsistent with constraints derived from narrowband H$\alpha$ imaging of theionized gas. The spatial coincidence between HI tail and depolarizationanalyzed in this paper could be the first observed evidence of a magnetic fieldthat either has passed through a radio galaxy lobe or has survived the lobeexpansion.

A depolarizing HI tidal tail in the western lobe of Fornax A