Characterizing magnetohydrodynamic turbulence in the Small Magellanic Cloud.
BURKHART B., STANIMIROVIC S., LAZARIAN A. and KOWAL G.
Abstract (from CDS):
We investigate the nature and spatial variations of turbulence in the Small Magellanic Cloud (SMC) by applying several statistical methods on the neutral hydrogen (H I) column density image of the SMC and a database of isothermal numerical simulations. By using the third and fourth statistical moments we derive the spatial distribution of the sonic Mach number Ms across the SMC. We find that about 90% of the H I in the SMC is subsonic or transonic. However, edges of the SMC "bar" have Ms ∼ 4 and may be tracing shearing or turbulent flows. Using numerical simulations we also investigate how the slope of the spatial power spectrum depends on both sonic and Alfvén Mach numbers. This allows us to gauge the Alfvén Mach number of the SMC and conclude that its hydrodynamic pressure dominates over the magnetic pressure. The trans-Alfvénic nature of the H I gas in the SMC is also highlighted by the bispectrum, a three-point correlation function which characterizes the level of non-Gaussianity in wave modes. We find that the bispectrum of the SMC H I column density displays similar large-scale correlations as numerical simulations; however, it has localized enhancements of correlations. In addition, we find a break in correlations at a scale of ∼160 pc. This may be caused by numerous expanding shells of a similar size.