Turbulent universal galactic Kolmogorov velocity cascade over 6 decades

TL;DR

This paper provides evidence for a universal, continuous turbulent velocity cascade across six orders of magnitude in galactic interstellar media, using a novel separation technique applied to spectroscopic data.

Contribution

It introduces an innovative method to isolate velocity fluctuations and demonstrates the existence of a universal Kolmogorov -5/3 velocity cascade in the interstellar medium across multiple phases.

Findings

Universal velocity cascade observed over 6 decades in scale.

Density fluctuations do not show universality and vary with phase.

Turbulent density fluctuations align with compressible turbulence theory.

Abstract

We report the evidence for the existence of the universal, continuous turbulent cascade of velocity fluctuations with Kolmogorov -5/3 slope spanning 6 orders of length scales, from $10^4$ pc down to $10^{-2}$ pc. This was achieved by applying our innovative technique of separating density and velocity fluctuations to a set of spectroscopic surveys featuring various galactic spectral lines. This unified velocity cascade involves different interstellar phases from diffuse galactic media to dense self-gravitating clouds and persists despite interstellar phase transitions. However, the turbulent density fluctuations do not show this universality as the value of the spectral slope changes in different interstellar phases. This agrees with the expectation of compressible turbulence theory and demonstrates that the density is only an indirect tracer of interstellar turbulence. We report that the density fluctuations for clouds and filaments that are preferentially parallel to magnetic fields exhibit the spectral slope of -2. The universal grand turbulent velocity cascade that is established in our paper has significant implications for key galactic physical processes, including star formation, cosmic ray transport etc. We anticipate our result to be a starting point for in vitro models of multiphase interstellar turbulence studies with a significant impact for modeling of spiral galaxies.