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

TL;DR

This study uses radio observations to analyze a depolarized HI tidal tail in Fornax A, revealing magnetic field properties and suggesting the tail's magnetic field has interacted with or survived the radio lobe environment.

Contribution

It provides the first evidence of a magnetic field in a tidal HI tail associated with a radio galaxy lobe, combining RM analysis with HI and Hα observations.

Findings

The HI tail exhibits a magnetic field strength of approximately 9.5-11 μG.

The observed RM excess indicates a magnetized medium associated with the HI tail.

The magnetic field in the tail is consistent with constraints from Hα imaging.

Abstract

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.