Faraday Rotation Structure on Kiloparsec Scales in the Giant Radio Lobes of Centaurus A

TL;DR

This study uses background radio sources to map the Faraday rotation structure of Centaurus A's giant radio lobes, revealing turbulence and magnetic field properties on kiloparsec scales for the first time.

Contribution

It introduces a novel application of background source RM analysis to radio galaxy lobes, providing new insights into their magnetic turbulence and structure.

Findings

Detected turbulent RM signal with 17 rad/m^2 rms in the southern lobe.

Identified a possible thin skin with coherent magnetic fields around the lobe.

Estimated a magnetic field reversal scale of about 20 kpc.

Abstract

We present the results of an Australia Telescope Compact Array 1.4 GHz spectropolarimetric aperture synthesis survey of 34 square degrees centred on Centaurus A - NGC 5128. A catalogue of 1005 extragalactic compact radio sources in the field to a continuum flux density of 3 mJy/beam is provided along with a table of Faraday rotation measures (RMs) and linear polarised intensities for the 28 percent of sources with high signal-to-noise in linear polarisation. We use the ensemble of 281 background polarised sources as line-of-sight probes of the structure of the giant radio lobes of Centaurus A. This is the first time such a method has been applied to radio galaxy lobes and we explain how it differs from the conventional methods that are often complicated by depth and beam depolarisation effects. We use an RM structure function analysis and report the detection of a turbulent RM signal, with rms of 17 rad/m^2 and scale size 0.3 degrees, associated with the southern giant lobe. We cannot verify whether this signal arises from turbulent structure throughout the lobe or only in a thin skin (or sheath) around the edge, although we favour the latter. The RM signal is modelled as possibly arising from a thin skin with a thermal plasma density equivalent to the Centaurus intragroup medium density and a coherent magnetic field that reverses its sign on a spatial scale of 20 kpc.