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

Magnetic turbulence is anisotropic because 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 Alfven, slow, and fast modes and studied how each of them contributes to the observed anisotropy. We found that when the angle between the line of sight and the mean magnetic field is large the Alfven mode dominates the observed anisotropy, while for smaller angles the anisotropy is not large enough to be used to probe the magnetization of the medium, and it is dominated by the slow mode. Our results are in fair agreement with the theoretical predictions in Kandel et al.