Modeling multiphase galactic outflows: A multifluid moving-mesh approach

Outflows are a key part of the galactic gas cycle and crucial in shaping the star formation activity in their host galaxy. In simulations of galaxy evolution, however, modeling these outflows in their multiphase nature and over the relevant timescales is an unsolved problem. We present a subgrid model for simulating multiphase galactic outflows in efficient, comparatively low-resolution simulations, designed for application in future cosmological simulations. The cold phase (T=10^4 K) is treated as pressureless, and its interaction with the hot phase is captured through source terms representing drag and mixing. These terms are obtained using analytic drag and mixing terms for single clouds and convolving them with a cloud mass distribution consistent with high-resolution simulations. Applied to a setup resembling the starburst galaxy M82, the model reproduces the velocity, density, and mass outflow rates of high-resolution simulations that resolve individual cold clouds. Cold outflows emerge naturally from interactions between the hot wind and cold interstellar clouds, with drag and mixing both contributing to the acceleration. Variation of the mixing strength strongly affects outflow properties: stronger mixing enhances mass transfer from hot to cold gas, reduces the hot-phase velocity, and accelerates the cold phase, while also affecting the origin and composition of the cold outflow. Weak mixing produces cold gas mostly from preexisting interstellar clouds, whereas stronger mixing leads to substantial cold gas formation from the hot phase. This framework enables efficient simulations of multiphase galactic outflows while retaining key multicomponent features of the outflow dynamics.