Statistical properties of Faraday rotation measure from large-scale magnetic fields in intervening disc galaxies

TL;DR

This paper models the probability distribution of Faraday rotation measures caused by large-scale magnetic fields in distant disc galaxies, enabling estimation of their magnetic field strengths from observational data.

Contribution

It introduces a statistical framework linking RM distribution width to galaxy magnetic field strength, providing a method to measure large-scale fields with ~50% accuracy.

Findings

RM distribution width correlates with mean magnetic field strength.

The model allows magnetic field estimation with about 50% accuracy.

Applicable when RM contributions from intervening galaxies are isolated.

Abstract

To constrain the large-scale magnetic field strengths in cosmologically distant galax- ies, we derive the probability distribution function of Faraday rotation measure (RM) when random lines of sight pass through a sample of disc galaxies, with axisymmetric large-scale magnetic fields. We find that the width of the RM distribution of the galaxy sample is directly related to the mean large-scale field strength of the galaxy population, provided the dispersion within the sample is lower than the mean value. In the absence of additional constraints on parameters describing the magneto-ionic medium of the intervening galaxies, and in the situation where RMs produced in the intervening galaxies have already been statistically isolated from other RM contributions along the lines of sight, our simple model of the magneto-ionic medium in disc galaxies suggests that the mean large-scale magnetic field of the population can be measured to within ~ 50% accuracy.