Large-scale turbulence cascade in the spiral galaxy NGC~6946

TL;DR

This study investigates large-scale turbulence in the spiral galaxy NGC 6946, revealing a power-law behavior in density and velocity spectra that suggests a combined influence of magnetic fields and gravity on turbulence driving mechanisms.

Contribution

It provides the first detailed measurement of turbulence power spectra at kiloparsec scales in an external galaxy using the VME method with new VLA observations.

Findings

Power spectra follow a power law with slopes of -0.96 and -1.81.

Turbulence is driven by both solenoidal and compressive forces.

Energy cascade occurs at scales larger than 6 kpc.

Abstract

The generation mechanism of compressible fluid turbulence at kiloparsec scales in the Interstellar Medium (ISM) 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 (VME) to measure the \HI 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 \HI Nearby Galaxy Survey (THINGS). The estimated power spectra are obeying a power law with a slope of $-0.96\pm0.05$ in column density and $-1.81\pm0.07$ 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 in 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.