Broadband radio polarimetry of Fornax A, I: Depolarized patches generated by advected thermal material from NGC 1316

TL;DR

This study uses broadband radio polarimetry to analyze low-polarization patches in Fornax A's lobes, revealing they are caused by complex Faraday effects from advected thermal plasma structured in shells or filaments.

Contribution

It demonstrates how broadband polarimetric techniques can resolve and interpret low-polarization patches as signatures of magnetized thermal plasma structures in radio galaxy lobes.

Findings

Low-$p$ patches are linked to magnetic field interfaces and Faraday effects.

Thermal plasma structures are likely advected from NGC 1316's ISM or ICM.

Spatial correlations reveal complex magnetic and plasma interactions.

Abstract

We present observations and analysis of the polarized radio emission from the nearby radio galaxy Fornax A over 1.28--3.1 GHz, using data from the Australia Telescope Compact Array (ATCA). In this, the first of two associated papers, we use modern broadband polarimetric techniques to examine the nature and origin of conspicuous low-polarization (low-$p$) patches in the lobes. We resolve the low-$p$ patches, and find that their low fractional polarization is associated with complicated frequency-dependent interference in the polarized signal generated by Faraday effects along the line of sight. The low-$p$ patches are spatially correlated with interfaces in the magnetic structure of the lobe, across which the line-of-sight-projected magnetic field changes direction. Spatial correlations with the sky-projected magnetic field orientation and structure in total intensity are also identified and discussed. We argue that the low-$p$ patches, along with associated reversals in the line-of-sight magnetic field and other related phenomena, are best explained by the presence of $\mathcal{O}(10^9)$ $M_\odot$ of magnetized thermal plasma in the lobes, structured in shells or filaments, and likely advected from the ISM of NCG 1316 or its surrounding ICM. Our study underscores the power and utility of spatially-resolved, broadband, full-polarization radio observations to reveal new facets of flow behaviors and magneto-ionic structure in radio lobes and their interplay with the surrounding environment.