Interstellar Turbulent Flows: Decoupled, Multiphase, and Multiphysics

Interstellar gas typically has Reynolds number far exceeding unity, as dynamic viscosities are 10^-4 to 10² g/(cm sec), while typical length scales exceed parsecs (3 x 10¹⁸ cm), typical velocities are 10 to 100 km/s, and typical densities are 10^-25 to 10^-22 g cm^-3. Thus, nearly all interstellar flows will develop turbulent structure. However, interstellar gas is extremely compressible, has multiple thermal phases, is coupled to magnetic fields, and is strongly influenced by gravity. Flows in the gas are driven both by gravity at all scales, and point-source stellar feedback (supernova explosions, ionizing radiation, and stellar outflows). In this talk I explore the implications of the deviation of interstellar gas from ideal incompressible turbulence models, with a focus on star formation in galaxies and the growth of magnetic fields due to the small-scale dynamo. In the case of gravitational collapse to star formation, the multiphase nature of the interstellar gas prevents large-scale turbulent flows driven by supernovae and other stellar feedback from effectively coupling to the turbulent flows driven by gravitational collapse within the dense, cold clouds where stars form. Only in extremely high surface density galaxies characteristic of the early universe are the large-scale flows also driven by gravity. In all cases, the density structure of star forming regions is ultimately determned by gravity despite the presence of turbulent flow. In the case of the small-scale dynamo, the thermal phase structure leads to decoupling of dynamos in different phases that grow at rates determined by the local sound speed. When there is a large filling factor of hot gas with high sound speed, magnetic energy growth occurs fastest there despite its low inertia. At other times, slower growth rates in cold or warm gas can still lead to substantial growth of total magnetic energy.