Simulated Faraday Rotation Measures toward High Galactic Latitudes

TL;DR

This study models the Galactic magnetic field's impact on Faraday rotation measures at high latitudes, revealing discrepancies with observations and suggesting additional factors like intergalactic fields or unmodeled magnetic structures.

Contribution

It introduces comprehensive simulations of the Galactic magnetic field's effect on Faraday RMs, highlighting limitations of current models in explaining observed data.

Findings

Observed RMs' medians are inconsistent with models.

Observed RM variability exceeds simulations.

Discrepancies suggest additional magnetic components or intergalactic contributions.

Abstract

We study the Faraday rotation measure (RM) due to the Galactic magnetic field (GMF) toward high Galactic latitudes. The RM arises from the global, regular component as well as from the turbulent, random component of the GMF. We model the former based on observations and the latter using the data of magnetohydrodynamic turbulence simulations. For a large number of different GMF models, we produce mock RM maps around the Galactic poles and calculate various statistical quantities with the RM maps. We find that the observed medians of RMs toward the north and south Galactic poles, $\sim 0.0\pm 0.5 {\rm rad m^{-2}}$ and $\sim +6.3\pm 0.5 {\rm rad m^{-2}}$, are difficult to explain with any of our many alternate GMF models. The standard deviation of observed RMs, $\sim 9 {\rm rad m^{-2}}$, is clearly larger than that of simulated RMs. The second-order structure function of observed RMs is substantially larger than that of simulated RMs, especially at small angular scales. We discuss other possible contributions to RM toward high Galactic latitudes. Besides observational errors and the intrinsic RM of background radio sources against which RM is observed, we suggest that the RM due to the intergalactic magnetic field may account for a substantial fraction of the observed RM. Finally we note that reproducing the observed medians may require additional components or/and structures of the GMF that are not present in our models.