Studying ISM magnetic fields and turbulent regimes from polarimetric maps

TL;DR

This study uses MHD simulations to analyze polarimetric maps of molecular clouds, revealing how turbulence affects magnetic field measurements and proposing a modified method to estimate magnetic field strength.

Contribution

The paper introduces a set of MHD simulations with synthetic polarization maps and a new modified Chandrashekhar-Fermi method applicable across different orientations and turbulence regimes.

Findings

Polarization degree decreases in denser regions due to line-of-sight vector cancellation.

Statistical analysis distinguishes different turbulent regimes in molecular clouds.

The modified Chandrashekhar-Fermi method accurately estimates magnetic field strength regardless of orientation.

Abstract

Polarimeric maps have been used on the characterization of the magnetic field in molecular clouds. However, it is difficult to determine the 3-dimensional properties of these regions from the projected maps. In that case, numerical simulations can be used as benchmarks for polarimetric measurements, and evetually reveal more about the interplay of turbulence and the magnetic field lines. In this work we provide a number of MHD numerical simulations of turbulent molecular clouds and created their synthetic dust emission polarization maps, varying the direction of the observer. We determined the correlation of emission intensity and polarization degree for the simulated models. We were able to reproduce the decay on polarization degree at denser regions without any assumption regarding the properties of the dusty component. The anti-correlation arises from the simple cancelation of the polarization vectors along the line of sight. This effect is amplified within denser regions as the magnetic field configuration becomes more complex. We studied the probability distribution, the power spectrum and the structure function of the polarization angles. These statistical analysis revealed strong defferences depending on the turbulent regime (i.e. sub/supersonic and sub/super-Alfvenic). Therefore, these methods can be used on polarimetric observations to characterize the dynamics of molecular clouds. We also presented a modified Chandrashekhar-Fermi method to obtain the intensity of the local magnetic field. The proposed formulation showed no limitations regarding orientation or turbulent regime.