WSRT Faraday tomography of the Galactic ISM at \lambda ~ 0.86 m

TL;DR

This study uses Faraday tomography on Galactic radio polarization data at 0.86 meters to map magnetic fields, analyze unresolved Faraday depth peaks, and explore depolarization effects in the interstellar medium.

Contribution

It demonstrates the application of Faraday tomography to Galactic ISM, addressing missing large-scale structures and modeling magnetic field components along the line of sight.

Findings

Unresolved Faraday depth peaks indicate separation of emission and rotation regions.

Depolarization over the telescope beam creates narrow polarized intensity canals.

Magnetic field mapping reveals regions with decreased line-of-sight depolarization.

Abstract

We investigate the properties of the Galactic ISM by applying Faraday tomography to a radio polarization data set in the direction of the Galactic anti-centre. We address the problem of missing large-scale structure in our data, and show that this does not play an important role for the results we present. The main peak of the Faraday depth spectra in our data set is not measurably resolved for about 8% of the lines of sight. An unresolved peak indicates a separation between the regions with Faraday rotation and synchrotron emission. However, cosmic rays pervade the ISM, and synchrotron emission would therefore also be produced where there is Faraday rotation. We suggest that the orientation of the magnetic field can separate the two effects. By modelling the thermal electron contribution to the Faraday depth, we map the strength of the magnetic field component along the line of sight. Polarized point sources in our data set have rotation measures that are comparable to the Faraday depths of the diffuse emission in our data. Our Faraday depth maps show narrow canals of low polarized intensity. We conclude that depolarization over the telescope beam produces at least some of these canals. Finally, we investigate the properties of one conspicuous region in this data set and argue that it is created by a decrease in line-of-sight depolarization compared to its surroundings.