Kinematics and Dynamics of Multiphase Outflows in Simulations of the Star-forming Galactic Interstellar Medium

Galactic outflows produced by stellar feedback are known to be multiphase in nature. Observations and simulations indicate that the material within several kiloparsecs of galactic disk midplanes 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 interstellar medium simulation based on the TIGRESS framework, for solar neighborhood conditions. To evaluate the extent of interactions among the phases, we examine the validity of the "ballistic model," which predicts the trajectories of the warm phase (5050 K 5 x 105 K) phase. The large energy flux from the hot outflow, 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 onesA.V. received travel support from ITS, SERB, Government of India, and would like to thank Biman B. Nath and Prateek Sharma for useful discussions and encouragement. The work of C.-G.K. was partly supported by a grant from the Simons Foundation (CCA 528307, E.C.O.). The work of E.C.O. and C.-G.K. was partly supported by NASA ATP grant NNX17AG26G. L.A. acknowledges support from the Australian Research Council’s Discovery Projects and Future Fellowships funding schemes, awards DP190101258 and FT18010037