The contribution of neutral gas to Faraday tomographic data at low frequencies. A first extensive comparison between real and synthetic data

TL;DR

This study compares real LOFAR polarization data with synthetic models to understand how neutral gas influences Faraday tomography, revealing that neutral HI significantly contributes to polarized emission at low frequencies.

Contribution

It introduces a new simulation framework and a metric to analyze the impact of neutral gas on Faraday structures, enhancing understanding of the interstellar medium's magnetic properties.

Findings

Synthetic observations match real data in polarization and RM levels.

CNM correlates with Faraday structures, influenced by turbulence and magnetic field orientation.

Weak correlation between CNM and polarized emission suggests complex ISM interactions.

Abstract

LOFAR observations of diffuse interstellar polarization at meter wavelengths reveal intricate polarized intensity structures with an unexpected correlation with neutral HI filaments that could not be reproduced in simulations with low cold neutral medium (CNM) abundance. We investigate whether MHD simulations of thermally bi-stable neutral interstellar medium, with a range of CNM fraction, can reproduce the properties of the 3C196 field, the high Galactic latitude test field. Using 50 pc simulations with varying levels of turbulence and compressibility, we generated synthetic 21 cm and synchrotron observations, including instrumental noise and beam effects, for different line-of-sight orientations relative to the magnetic field. We developed MOOSE, a code to generate synthetic synchrotron polarization and Faraday tomography. We also developed a metric based on the HOG algorithm, to quantify the relative contribution of cold and warm neutral medium structures to the Faraday tomographic data. The synthetic observations show levels of polarization intensity and RM values comparable to the 3C196 field, indicating that thermal electrons associated with the neutral HI phase can account for a significant fraction of the synchrotron polarized emission at 100-200 MHz. The simulations consistently reveal a correlation between CNM and Faraday tomographic structures that depends on turbulence level, magnetic field orientation, and observational noise, but only weakly on CNM fraction. We found slightly weaker CNM-Synchrotron polarized emission correlation level than observed in the 3C196 field. These results suggest that low-frequency polarimetric observations provide a valuable probe of magnetic-field morphology in the multi-phase Solar-neighborhood ISM, while simultaneously underscoring the need for improved modeling of the turbulent, multi-phase, and partially ionized interstellar medium.