Faraday Rotation Measure Synthesis of intermediate redshift quasars as a probe of intervening matter

TL;DR

This study uses Faraday Rotation Measure Synthesis to analyze the magnetic properties of intervening matter in quasars at intermediate redshifts, revealing complex Faraday structures linked to inhomogeneous magnetic fields.

Contribution

It introduces high-resolution polarization measurements and a novel analysis of Faraday Depth spectra to distinguish between intrinsic and intervening magnetic effects in quasars.

Findings

Intervening MgII absorbers are associated with increased depolarization.

Faraday screens caused by intervening matter produce inhomogeneous magnetic fields with strengths of at least 3 μG.

Complex Faraday structures are influenced by both intervening material and intrinsic source properties.

Abstract

There is evidence that magnetized material along the line of sight to distant quasars is detectable in the polarization properties of the background sources. The polarization properties appear to be correlated with the presence of intervening MgII absorption, which is thought to arise in outflowing material from star forming galaxies. In order to investigate this further, we have obtained high spectral resolution polarization measurements, with the VLA and ATCA, of a set of 49 unresolved quasars for which we have high quality optical spectra. These enable us to produce a Faraday Depth spectrum for each source, using Rotation Measure Synthesis. Our new independent radio data confirms that interveners are strongly associated with depolarization. We characterize the complexity of the Faraday Depth spectrum using a number of parameters and show how these are related, or not, to the depolarization and to the presence of MgII absorption along the line of sight. We argue that complexity and structure in the Faraday Depth distribution likely arise from both intervening material and intrinsically to the background source and attempt to separate these. We find that the strong radio depolarization effects associated with intervening material at redshifts out to $z \approx 1$ arise from inhomogeneous Faraday screens producing a dispersion in Rotation Measure across individual sources of around 10~rad/m$^2$. This is likely produced by disordered fields with strengths of at least $3\;\mu$G.