A new method for spatially resolving the turbulence driving mixture in the ISM with application to the Small Magellanic Cloud

Gerrard, Isabella A; Federrath, Christoph; Pingel, Nickolas M; McClure-Griffiths, Naomi M; Marchal, Antoine; Joncas, Gilles; Clark, Susan E; Stanimirović, Snežana; Lee, Min-Young; van Loon, Jacco Th; Dickey, John; Dénes, Helga; Ma, Yik Ki; Dempsey, James; Lynn, Callum

Abstract

Turbulence plays a crucial role in shaping the structure of the interstellar medium. The ratio of the three-dimensional density contrast ($\sigma _{\rho /\rho _0}$) to the turbulent sonic Mach number ($\mathcal {M}$) of an isothermal, compressible gas describes the ratio of solenoidal to compressive modes in the turbulent acceleration field of the gas, and is parameterised by the turbulence driving parameter: $b=\sigma _{\rho /\rho _0}/\mathcal {M}$. The turbulence driving parameter ranges from b = 1/3 (purely solenoidal) to b = 1 (purely compressive), with b = 0.38 characterising the natural mixture (1/3 compressive, 2/3 solenoidal) of the two driving modes. Here we present a new method for recovering $\sigma _{\rho /\rho _0}$, $\mathcal {M}$, and b, from observations on galactic scales, using a roving kernel to produce maps of these quantities from column density and centroid velocity maps. We apply our method to high-resolution ${\rm H}\, \small {I}$ emission observations of the Small Magellanic Cloud (SMC) from the GASKAP-HI survey. We find that the turbulence driving parameter varies between b ∼ 0.3 and b ∼ 1.0 within the main body of the SMC, but the median value converges to b ∼ 0.51, suggesting that the turbulence is overall driven more compressively (b > 0.38). We observe no correlation between the b parameter and ${\rm H}\, \small {I}$ or Hα intensity, indicating that compressive driving of ${\rm H}\, \small {I}$ turbulence cannot be determined solely by observing ${\rm H}\, \small {I}$ or Hα emission density, and that velocity information must also be considered. Further investigation is required to link our findings to potential driving mechanisms such as star-formation feedback, gravitational collapse, or cloud-cloud collisions.

• Journal Article Type: Article
• Acceptance Date: Sep 6, 2023
• Online Publication Date: Sep 12, 2023
• Publication Date: 2023-11
• Deposit Date: Sep 28, 2023
• Publicly Available Date: Oct 2, 2023
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 526
• Issue: 1
• DOI: https://doi.org/10.1093/mnras/stad2718
• Keywords: Space and Planetary Science, Astronomy and Astrophysics