Faraday rotation as a probe of radio galaxy environment in RMHD AGN jet simulations

TL;DR

This paper demonstrates how Faraday rotation measurements in simulations can help distinguish the environments of radio galaxies, improving understanding of AGN feedback and overcoming degeneracies in observational data.

Contribution

It introduces a novel simulation-based approach using Faraday rotation to probe radio galaxy environments, addressing degeneracies in physical parameter interpretation.

Findings

Enhanced RMs near jet head and lobe edges

RM reversal across jet axis observed in simulations

Depolarisation frequency correlates with environmental properties

Abstract

Active galactic nuclei (AGN) play an integral role in galaxy formation and evolution by influencing galaxies and their environments through radio jet feedback. Historically, interpreting observations of radio galaxies and quantifying radio jet feedback has been challenging due to degeneracies between their physical parameters. In particular, it is well-established that different combinations of jet kinetic power and environment density can yield indistinguishable radio continuum properties, including apparent size and Stokes I luminosity. We present an approach to breaking this degeneracy by probing the line-of-sight environment with Faraday rotation. We study this effect in simulations of three-dimensional relativistic magnetohydrodynamic AGN jets in idealised environments with turbulent magnetic fields. We generate synthetic Stokes I emission and Faraday rotation measure (RM) maps, which enable us to distinguish between our simulated sources. We find enhanced RMs near the jet head and lobe edges and an RM reversal across the jet axis. We show that increasing the environment density and the average cluster magnetic field strength broadens the distribution of Faraday rotation measure values. We study the depolarisation properties of our sources, finding that the hotspot regions depolarise at lower frequencies than the lobes. We quantify the effect of depolarisation on the RM distribution, finding that the frequency at which the source is too depolarised to measure the RM distribution accurately is a probe of environmental properties. This technique offers a range of new opportunities for upcoming surveys, including probing radio galaxy environments and determining more accurate estimates of the AGN feedback budget.