Is there a polarization horizon?

TL;DR

This paper models polarized radio signals through the interstellar medium to understand depolarization effects and the complexity of Faraday depth spectra, revealing limitations in current observational interpretations.

Contribution

It introduces a detailed model of the interstellar medium with multiple components to simulate polarized emission and depolarization effects across a range of frequencies.

Findings

Faraday complexity exceeds observations in synthetic models.

Depolarization effects vary with the presence of H II regions and frequency.

Faraday depth can be randomized, obscuring true magnetic field information.

Abstract

Modern radio spectrometers make measurement of polarized intensity as a function of Faraday depth possible. I investigate the effect of depolarization along a model line of sight. I model sightlines with two components informed by observations: a diffuse interstellar medium with a lognormal electron density distribution and a narrow, denser component simulating a spiral arm or H~{\sc ii} region, all with synchrotron-emitting gas mixed in. I then calculate the polarized intensity from 300-1800~MHz and calculate the resulting Faraday depth spectrum. The idealized synthetic observations show far more Faraday complexity than is observed in Global Magneto-Ionic Medium Survey observations. In a model with a very nearby H~{\sc ii} region observed at low frequencies, most of the effects of a "depolarization wall" are evident: the H~{\sc ii} region depolarizes background emission and less (but not zero) information from beyond the H~{\sc ii} region reaches the observer. In other cases, the effects are not so clear, as significant amounts of information reach the observer even through significant depolarization, and it is not clear that low-frequency observations sample largely different volumes of the interstellar medium than high-frequency observations. The observed Faraday depth can be randomized such that it does not always have any correlation with the true Faraday depth.