KIPAC Seminar: Supernova remnants as engines of non-Kolmogorov galactic turbulence

Abstract: The plasma in our Galaxy is in a state of turbulence, influencing everything from the thermal phase structure of the interstellar medium (ISM) to the transport of cosmic rays and the scintillation of light through the Galaxy. Galactic turbulence can be readily sustained by the energy and momentum deposited by supernova explosions alone; yet we lack detailed characterizations of supernova-driven turbulence that would allow meaningful comparison with commonly assumed Kolmogorov models. In this seminar, I report on high-resolution, hydrodynamical, gravitationally stratified, supernova (SN)-driven, multiphase ISM simulations, which I use to probe the nature of the galactic turbulence cascade. Using velocity flux transfer functions decomposed into interactions between compressible and incompressible modes, I show that an inverse cascade from small to large scales is generated by supernova remnants (SNRs). This cascade feeds flux into scales well beyond the gaseous scale height, energizing galactic winds and fueling additional forward cascades. I show that the incompressible turbulence in these ISM models, to leading order, arises from baroclinicity generated at the unstable contact discontinuity between warm and hot plasma in SNRs, rather than from shock interactions, as is the conventional wisdom. As SNRs expand beyond their cooling radius, this contact discontinuity corrugates and becomes the primary engine for incompressible turbulence, driving incompressible velocity modes across a broad range of scales and generating a non-Kolmogorov velocity spectrum, k^{-3/2}. I derive analytical relations connecting this unstable layer to the turbulence it generates, showing that large-scale galactic turbulence may be developed and sustained by small-scale thin-shell instabilities, and demonstrating that galactic turbulence shares little in common with Kolmogorov-type turbulence.