Measuring magnetization with rotation measures and velocity centroids in supersonic MHD turbulence

TL;DR

This paper introduces a new method to measure magnetic field anisotropy in supersonic interstellar turbulence by analyzing rotation measures and velocity centroids, especially in low-density regions, supported by 3D MHD simulations.

Contribution

The study demonstrates that selecting low-density regions in supersonic MHD turbulence allows accurate measurement of magnetic field anisotropy, confirming the theoretical M_A^{-4/3} relation.

Findings

Anisotropy depends on Alfvénic Mach number as M_A^{-4/3}.

Selective sampling of low-density regions improves measurement accuracy.

Synthetic observations of CO emission support the theoretical relation.

Abstract

The interstellar turbulence is magnetized and thus anisotropic. The anisotropy of turbulent magnetic fields and velocities is imprinted in the related observables, rotation measures (RMs), and velocity centroids (VCs). This anisotropy provides valuable information on both the direction and strength of the magnetic field. However, its measurement is difficult especially in highly supersonic turbulence in cold interstellar phases due to the distortions by isotropic density fluctuations. By using 3D simulations of supersonic and sub-Alfv\'enic magnetohydrodynamic(MHD) turbulence, we find that the problem can be alleviated when we selectively sample the volume-filling low-density regions in supersonic MHD turbulence. Our results show that in these low-density regions, the anisotropy of RM and VC fluctuations depends on the Alfv\'enic Mach number as $\rm M_A^{-4/3}$. This anisotropy-$\rm M_A$ relation is theoretically expected for sub-Alfv 'enic MHD turbulence and confirmed by our synthetic observations of $^{12}$CO emission. It provides a new method for measuring the plane-of-the-sky magnetic fields in cold interstellar phases.