Turbulence driving in the diffuse interstellar medium

Abstract

Turbulence plays a fundamental role in regulating the structure, dynamics, and evolution of the interstellar medium (ISM). A key quantity in understanding how turbulence is driven in the ISM is the turbulence driving parameter, b, which characterizes the balance between solenoidal (divergence-free) and compressive (curl-free) driving modes. In this thesis, we investigate the spatial and temporal variation of b, the turbulent Mach number (Mach), and the volume density contrast (sigma_rho) across different ISM environments, combining observational data and numerical simulations.

We present a generalized method for mapping turbulence properties using column density and centroid velocity information, and apply this technique to high-resolution GASKAP-HI observations of the Small Magellanic Cloud (SMC). The turbulence driving parameter is found to vary between b ~ 0.3 and b ~1.0 across the SMC, with a median of b ~ 0.5, suggesting predominantly compressive driving. No clear correlation is found between b and HI or H-alpha intensity, highlighting the need for kinematic information in diagnosing turbulence driving mechanisms.

Extending this analysis to two extra-planar HI clouds observed with MeerKAT, we report the first observational measurements of turbulence in neutral hydrogen entrained in the Milky Way's nuclear wind. Both clouds exhibit sub-to-trans-sonic turbulence with b ~ 1, despite differing physical environments and galactic latitudes. These results suggest that compressive turbulence driving may be induced by the interaction between the wind and entrained atomic material, independent of in-situ star formation.

Finally, we examine the time evolution of b and related turbulence properties in the warm neutral medium of the TIGRESS magnetohydrodynamic simulations, which self-consistently model star formation, supernovae, radiation feedback, and gravity in a galaxy simulation that approximates the solar-neighbourhood-like ISM. Over a ~100 million year timescale, we find that b fluctuates between ~0.3 and ~0.8, with a time-dependent relationship to star formation activity. More compressive driving tends to precede peaks in star formation, while solenoidal driving becomes more prominent during and after supernova feedback events.

Together, these studies provide new insights into the driving and regulation of turbulence in the multiphase ISM, and lay the groundwork for future investigations connecting observed turbulence statistics to the underlying physical processes shaping galaxy evolution. Show more