Turbulent Driving Scales in Molecular Clouds

TL;DR

This study uses principal component analysis on simulated and real molecular cloud data to determine that turbulence is primarily driven at large scales, likely by processes like supernovae or spiral shocks, rather than small-scale internal sources.

Contribution

It provides the first quantitative constraint showing turbulence in molecular clouds is driven at large scales, based on comparison of MHD simulations and observations.

Findings

Large-scale driving models match observations

Small-scale internal sources cannot reproduce observed turbulence

Large-scale astrophysical processes are likely responsible for turbulence

Abstract

Supersonic turbulence in molecular clouds is a dominant agent that strongly affects the clouds' evolution and star formation activity. Turbulence may be initiated and maintained by a number of processes, acting at a wide range of physical scales. By examining the dynamical state of molecular clouds, it is possible to assess the primary candidates for how the turbulent energy is injected. The aim of this paper is to constrain the scales at which turbulence is driven in the molecular interstellar medium, by comparing simulated molecular spectral line observations of numerical magnetohydrodynamic (MHD) models and molecular spectral line observations of real molecular clouds. We use principal component analysis, applied to both models and observational data, to extract a quantitative measure of the driving scale of turbulence. We find that only models driven at large scales (comparable to, or exceeding, the size of the cloud) are consistent with observations. This result applies also to clouds with little or no internal star formation activity. Astrophysical processes acting on large scales, including supernova-driven turbulence, magnetorotational instability, or spiral shock forcing, are viable candidates for the generation and maintenance of molecular cloud turbulence. Small scale driving by sources internal to molecular clouds, such as outflows, can be important on small scales, but cannot replicate the observed large-scale velocity fluctuations in the molecular interstellar medium.