Characterizing Magnetohydrodynamic Turbulence in the Small Magellanic Cloud

TL;DR

This study analyzes the turbulence characteristics of the Small Magellanic Cloud's neutral hydrogen gas, revealing mostly subsonic or transonic turbulence with localized regions of high Mach number and magnetic influence, using statistical and simulation methods.

Contribution

It provides the first detailed spatial mapping of turbulence parameters in the SMC using advanced statistical techniques and numerical simulations, highlighting the dominance of gas pressure over magnetic pressure.

Findings

90% of HI in the SMC is subsonic or transonic

Edges of the SMC bar show high Mach numbers up to 4

The gas is super-Alfvenic, with magnetic pressure less dominant

Abstract

We investigate the nature and spatial variations of turbulence in the Small Magellanic Cloud (SMC) by applying several statistical methods on the neutral hydrogen (HI) column density image of the SMC and a database of isothermal numerical simulations. By using the 3rd and 4th statistical moments we derive the spatial distribution of the sonic Mach number (M_s) across the SMC. We find that about 90% of the HI in the SMC is subsonic or transonic. However, edges of the SMC `bar' have M_s=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 Alfven Mach numbers. This allows us to gauge the Alfven Mach number of the SMC and conclude that its gas pressure dominates over the magnetic pressure. The super-Alfvenic nature of the HI 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 HI 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.