Kinematics and Dynamics of Multiphase Outflows in Simulations of the Star-Forming Galactic ISM

Abstract: Galactic outflows produced by stellar feedback are known to be multiphase in nature. Both observations and simulations indicate that the material within several kpc of galactic disk mid-planes consists of warm clouds embedded within a hot wind. A theoretical understanding of the outflow phenomenon, including both winds and fountain flows, requires study of the interactions among thermal phases. We develop a method to quantify these interactions via measurements of mass, momentum, and energy flux exchanges using temporally and spatially averaged quantities and conservation laws. We apply this method to a star-forming ISM MHD simulation based on the TIGRESS framework, for Solar neighbourhood conditions. To evaluate the extent of interactions among the phases, we first examine the validity of the ``ballistic model,'' which predicts trajectories of the warm phase ($5050\,\rm{K}<T\<2\times10^4\,\rm{K}$) treated as non-interacting clouds. This model is successful at intermediate vertical velocities ($ 50$ km s$^{-1}$ $\lesssim |v_z| \lesssim 100 $ km s$^{-1}$), but at higher velocities we observe an excess in simulated warm outflow compared to the ballistic model. This discrepancy cannot be fully accounted for by cooling of high-velocity intermediate-temperature ($2\times10^4\,\rm{K}<T\<5\times10^5\,\rm{K}$) gas. By examining the fluxes of mass, momentum and energy, we conclude that warm phase gains mass via cooling of the intermediate phase, while momentum transfer occurs from the hot ($T\>5\times10^5\,\rm{K}$) to the warm phase. The large energy flux from the hot outflow that is transferred to the warm and intermediate phases is quickly radiated away. A simple interaction model implies an effective warm cloud size in the fountain flow of a few 100~pc, showing that warm-hot flux exchange mainly involves a few large clouds rather than many small ones.