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 tests whether state-of-the-art MHD simulations of thermally bistable neutral ISM can reproduce LOFAR Faraday tomography of the 3C196 field. By generating synthetic 21 cm and synchrotron data with MOOSE and analyzing CNM–WNM correlations via a Histogram-of-Oriented-Gradients metric, the authors show that thermal electrons in HI can contribute substantially to low-frequency polarization and Faraday structures. The CNM–Faraday correlation exists in the simulations and depends on turbulence and LOS orientation, but the exact CNM-dominant signal seen in 3C196 is not fully reproduced, suggesting missing physics (e.g., WIM, ionization prescriptions, or more realistic forcing). The work highlights low-frequency polarimetry as a diagnostic of magnetic-field morphology in the local multi-phase ISM while calling for broader comparisons and improved turbulent-ISM modeling to fully capture observed correlations.

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.

Figures (23)

• Figure 1: Moment 0 (integrated polarized intensity) de-biased for noise (top), Moment 1 (middle) and Moment 2 (bottom) of the 3C196 field from LOFAR LoTSS DR2 Faraday tomographic data Erceg2022.
• Figure 2: Slice through the simulation cube showing the logarithmic gas density overlaid with magnetic field orientation using the line integral convolution (LIC) technique. Left: projection along the mean magnetic field direction; Right: projection perpendicular to the mean field.
• Figure 3: CNM mass fraction $\mathrm{f_{\textrm{CNM}}}$ of the different simulations used in this study. The dashed circle encircle the simulations with the same turbulent modes ratio $\mathrm{\chi}$. Colors show the initial input for the forcing amplitude.
• Figure 4: $M_1$ distributions as a function of $f_{\mathrm{CNM}}$ for the set of simulations. These distributions are measured along two orthogonal directions: parallel (lighter color) and perpendicular (darker color) to the mean magnetic field. Filled circles indicate the corresponding mean values $\langle M_1 \rangle$, outlined in black. The case-study simulation is highlighted in red, while the remaining simulations are shown in violet. Two simulations have $f_\textrm{CNM} = 26 \%$, leading to an over-plotting of their respective distributions. The 3C196 field is displayed in blue, with its associated distribution and mean value.
• Figure 5: Simulation 7. $RM$ histograms for cubes produced with the line-of-sight parallel, perpendicular or at $45^\circ$ with the mean direction of $\vec{B}$. Dashed lines: the mean values of the rotation measure maps. The mean values are $\langle RM_\parallel\rangle = 5.5~\textrm{rad~m}^{-2}$, $\langle RM_\perp\rangle = 0.0~\textrm{rad~m}^{-2}$, $\langle RM_{45^\circ}\rangle = 3.9~\textrm{rad~m}^{-2}$.