Large-scale turbulence cascade in the spiral galaxy NGC 6946

Abstract

The generation mechanism of compressible fluid turbulence at kiloparsec scales in the interstellar medium is a long-lasting puzzle. In this work, we explore the nature of large-scale turbulence in the external spiral galaxy NGC 6946. We use the visibility moment estimator to measure the H i column density and line-of-sight turbulent velocity power spectra combining the new observations of A array configuration of Karl G. Jansky Very Large Array (VLA) with the VLA B, C, D array observations from The H i Nearby Galaxy Survey. The estimated power spectra are obeying a power law with a slope of -0.96 ± 0.05 in column density and -1.81 ± 0.07 in line-of-sight velocity in length-scales ranging from 6 kpc to 170 pc. This points towards a forward energy cascade in the plane of the disc with a driving scale at least as large as 6 kpc. The values of the power-law indices indicate a combination of solenoidal and compressive force responsible for driving the measured turbulence. The presence of strong regular magnetic fields from the magnetic spiral arms in the galaxy is possibly contributing to the solenoidal part, while self-gravity or gravitational instability can mostly be the input for the compressive part of the forcing in the driving mechanism.