Measuring the Alfvenic Nature of the Interstellar Medium: Velocity Anisotropy Revisited

TL;DR

This paper refines a method to estimate the magnetic influence in the interstellar medium by analyzing velocity anisotropy, validated with simulations considering observational effects, enhancing understanding of turbulence and magnetic fields.

Contribution

It extends the anisotropy-based Alfven Mach number estimation method by testing its robustness against various observational effects and different centroid calculations.

Findings

Velocity anisotropy correlates with magnetic field strength and orientation.

Anisotropy is less prominent when the line-of-sight is within ~45 degrees of the magnetic field.

Radiative transfer effects do not significantly alter the anisotropy measurements.

Abstract

The dynamics of the interstellar medium (ISM) are strongly affected by turbulence, which shows increased anisotropy in the presence of a magnetic field. We expand upon the Esquivel & Lazarian method to estimate the Alfven Mach number using the structure function anisotropy in velocity centroid data from position-position-velocity maps. We utilize 3D magnetohydrodynamic (MHD) simulations of fully developed turbulence, with a large range of sonic and Alfvenic Mach numbers, to produce synthetic observations of velocity centroids with observational characteristics such as thermal broadening, cloud boundaries, noise, and radiative transfer effects of carbon monoxide. In addition, we investigate how the resulting anisotropy-Alfven Mach number dependency found in Esquivel & Lazarian (2011) might change when taking the second moment of the position-position-velocity cube or when using different expressions to calculate the velocity centroids. We find that the degree of anisotropy is related primarily to the magnetic field strength (i.e. Alfven Mach number) and the line-of-sight orientation, with a secondary effect on sonic Mach number. If the line-of-sight is parallel to up to ~45 deg off of the mean field direction, the velocity centroid anisotropy is not prominent enough to distinguish different Alfvenic regimes. The observed anisotropy is not strongly affected by including radiative transfer, although future studies should include additional tests for opacity effects. These results open up the possibility of studying the magnetic nature of the ISM using statistical methods in addition to existing observational techniques.