Velocity Centroids Anisotropy and the Signature of different MHD Modes in the Turbulent ISM

TL;DR

This study uses MHD simulations to analyze how different MHD wave modes contribute to anisotropy in velocity centroid observations, revealing the dominant modes depending on the viewing angle relative to the magnetic field.

Contribution

It distinguishes the contributions of Alfvén, slow, and fast MHD modes to velocity centroid anisotropy in the turbulent interstellar medium using synthetic observations.

Findings

Alfvén mode dominates anisotropy at large angles to magnetic field

Slow mode influences anisotropy at smaller angles

Results agree with theoretical predictions by Kandel et al.

Abstract

Magnetic turbulence is anisotropic as the directions of motion are constrained by the magnetic field. Such anisotropy can be observed in velocity centroids obtained from spectroscopic observations. We use magnetohydrodynamics (MHD) simulations to produce synthetic spectroscopic observations (position-position-velocity data) and study the anisotropy in the structure function of velocity centroid maps. We decomposed the velocity in the simulations into Alfv\'en, slow and fast-modes and studied how each of them contribute to the observed anisotropy. We found that when the angle between the line of sight and the mean magnetic field is large the Alfv\'en-mode dominates the observed anisotropy, while for smaller angles the anisotropy is not large enough to be used to probe the magnetization of the media, and it is dominated by the slow-mode. Our results are in fair agreement with the theoretical predictions in Kandel et al.(2016,2017).