A depolarizing H I tidal tail in the western lobe of Fornax A

Recent MeerKAT neutral hydrogen (H I) 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 H I 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 H I tail and in its neighborhood. We modeled the observed RM structure function to investigate the magnetic field power spectrum at the location of the H I tail and in a nearby control region. We found that the observed RM, in the control region and in a region enclosing the H I 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 mu G can explain the control region RM, but it is clear that there is an excess in correspondence with the H I tail region. We evaluated several scenarios in which the H I tail is either in the lobe foreground or embedded in the lobe. We determined a magnetic field strength on the order of similar to 9.5-11 mu G in the H I tail, a value consistent with constraints derived from narrowband H alpha imaging of the ionized gas. The spatial coincidence between H I 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.