The turbulence driving mode in NGC7793 and NGC1313

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.

Figures (9)

• Figure 1: Optical and peak temperature ($T_\mathrm{peak}$) images of NGC7793. Left: Optical image with CO ($J$=2--1) peak temperature contours overlayed. The optical image combines Hubble Space Telescope (HST) images from the Legacy Extragalctic UV Survey (LEGUS) images LEGUS_2015. The three-colour composite image combines the UVES/F555W (blue), UVES/F438W (green) and UVES/F255W (red). The contours are the peak temperature from the ALMA CO ($J$=2--1) emission. Right: The peak temperature map of the CO ($J$=2--1) emission. This is the same as the contours overlaying the left-hand image with a shared colorbar. The white dashed square in both images is the subsection of the galaxy used as an example in Sec.~\ref{['sec:methodology']}.
• Figure 2: Same as Fig. \ref{['fig:NGC7793_OptImage']}, but for NGC1313. The optical image combines HST images from the LEGUS survey where the images are coloured such that the UVES/F814W is blue, UVES/F555W is green and UVES/F275W filter is red LEGUS_2015.
• Figure 3: Example demonstration of the steps in the turbulent column density analysis for a subsection of NGC7793 (shown as the dashed square in Fig. \ref{['fig:NGC7793_OptImage']}). Panels (i)--(ix) show $M_0$ without S/N threshold (i), $M_0$ with S/N threshold of 5 (ii), CO peak temperature map (iii), CO-to-H$_2$ conversion factor, $X_\text{CO}$ (iv), column density, $\Sigma=X_\text{CO} M_0$ (v), linearly interpolated column density (vi), mean-normalised and Gaussian-smoothed column density (vii), turbulence-isolated column density (viii), and the same as (viii), but with the S/N mask reapplied (ix). The dashed circle indicates the kernel FWHM of 5 beam widths ($65\,\mathrm{pc}$ in diameter).
• Figure 4: PDFs of the logarithm of the normalised column density, $\Sigma/\langle\Sigma\rangle$ (blue histogram) compared to the turbulence-isolated version of $\Sigma/\langle\Sigma\rangle$ (red histogram) of the kernel section shown in Fig. \ref{['fig:Mom0_kernel_subtr']}. The shape of the distribution becomes more Gaussian and the standard deviation is reduced after turbulence isolation, as the non-turbulent column density fluctuations are removed by the turbulence-isolation procedure (see Sec. \ref{['sec:LowPassFilter']}).
• Figure 5: Maps of the CO ($J$=2--1) peak temperature, $T_\mathrm{peak}$, as in Figs. \ref{['fig:NGC7793_OptImage']} and \ref{['fig:NGC1313_OptImage']} (1st row), turbulent density dispersion, $\sigma_{\rho/\langle\rho\rangle}$ (2nd row), Mach number, $\mathcal{M}$ (3rd row), and turbulence driving parameter, $b$ in NGC7793 (left) and NGC1313 (right). The white contours in the top panels outline regions in which the turbulence analysis yielded valid results based on several criteria (see Sec. \ref{['sec:RovKern']}). The dashed contours define regions defined in Finn_2024_b, as annotated: the Centre, Ring and Outer regions in NGC7793, and the North Arm, Bar, South Arm, and Interarm regions in NGC1313. The galaxy centres are marked with a cross. While the turbulence analysis can only be successfully performed in a few local regions (hence the sparseness of the turbulence data), they span across the entire field of view of both galaxies, and show substantial spatial variation.