Kavli Affiliate: Susan E. Clark
| First 5 Authors: Isabella A. Gerrard, Christoph Federrath, Nickolas M. Pingel, Naomi M. McClure-Griffiths, Antoine Marchal
| Summary:
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 HI
emission observations of the Small Magellanic Cloud (SMC) from the GASKAP-HI
survey. We find that the turbulence driving parameter varies between $bsim
0.3$ and $bsim 1.0$ within the main body of the SMC, but the median value
converges to $bsim0.51$, suggesting that the turbulence is overall driven more
compressively ($b>0.38$). We observe no correlation between the $b$ parameter
and HI or H$alpha$ intensity, indicating that compressive driving of HI
turbulence cannot be determined solely by observing HI or H$alpha$ 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.
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