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

Abstract

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 parameterized 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 characterizing 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 ${\rm H}\,\rm{\small I}$ 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 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 ${\rm H}\,\rm{\small I}$ or H α intensity, indicating that compressive driving of ${\rm H}\,\rm{\small I}$ turbulence cannot be determined solely by observing ${\rm H}\,\rm{\small I}$ or H α 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.