The turbulence driving mode in NGC7793 and NGC1313

TL;DR

This study maps turbulence driving modes across two galaxies using ALMA data, revealing predominantly solenoidal turbulence with spatial variations linked to galactic environment and morphology.

Contribution

It applies a kernel-based analysis to measure turbulence driving parameters across entire galactic discs, demonstrating environmental dependence of turbulence modes.

Findings

NGC1313 shows higher Mach numbers and density dispersions than NGC7793.

Turbulence is predominantly solenoidal but varies spatially within galaxies.

Turbulence driving mode correlates with galactic radius and morphology.

Abstract

We present spatially resolved measurements of turbulence driving modes across entire extragalactic discs of NGC7793 and NGC1313, using Atacama Large Millimetre/submillimetre Array (ALMA) CO(J=2-1) observations at 13pc resolution. By applying a kernel-based analysis of density and velocity fluctuations, we map the turbulence driving parameter, b, which characterises the balance between solenoidal ($b\sim0.3$) and compressive ($b\sim1$) turbulent driving regimes. b is quantified as the ratio of the turbulent density fluctuations relative to the turbulent sonic Mach number, M. Both galaxies show predominantly solenoidal driving on average for the regions where we find valid results ($b\geq 0.33(\pm 0.05)^{+0.14}_{-0.10}$ in NGC7793; $b\geq 0.24(\pm 0.03)^{+0.10}_{-0.07}$ in NGC1313), noting that this is without including the influences of magnetic fields, making these measurements lower limits. We find substantial spatial variation of b, including localised regions of strongly compressive driving. NGC1313 exhibits higher turbulent Mach numbers and density dispersions than NGC7793, consistent with the disturbed morphology and recent satellite interaction in NGC1313. The turbulence in both NGC7793 and NGC1313 is supersonic ($3\lesssim M\lesssim 20$), and NGC1313 shows a radially decreasing trend of M with galactocentric radius. Radial trends indicate more solenoidal driving in the galaxy centres, potentially reflecting enhanced shear, and increasingly compressive modes in the outskirts. These results demonstrate that turbulence driving varies systematically with galactic environment and cannot be assumed uniform across discs. Our study applies a previously established method to larger scales and new data, linking local turbulence physics to global star formation regulation in galaxies, providing a new avenue for testing theoretical models with future integral field units and ALMA surveys.