Statistics of Centroids of Velocity

TL;DR

Velocity centroid statistics are useful for studying subsonic interstellar turbulence and magnetic field directions, but are limited in supersonic turbulence due to density fluctuation influences.

Contribution

This paper reviews the applicability and limitations of velocity centroid statistics in analyzing interstellar turbulence, highlighting their effectiveness in subsonic regimes and their inability to accurately trace supersonic turbulence.

Findings

Velocity centroids effectively probe subsonic turbulence.

They do not reliably trace supersonic turbulence due to density fluctuations.

Anisotropy in two-point statistics reveals magnetic field directions.

Abstract

We review the use of velocity centroids statistics to recover information of interstellar turbulence from observations. Velocity centroids have been used for a long time now to retrieve information about the scaling properties of the turbulent velocity field in the interstellar medium. We show that, while they are useful to study subsonic turbulence, they do not trace the statistics of velocity in supersonic turbulence, because they are highly influenced by fluctuations of density. We show also that for sub-Alfv\'enic turbulence (both supersonic and subsonic) two-point statistics (e.g. correlation functions or power-spectra) are anisotropic. This anisotropy can be used to determine the direction of the mean magnetic field projected in the plane of the sky.