First Results from SMAUG: Characterization of Multiphase Galactic Outflows from a Suite of Local Star-forming Galactic Disk Simulations

Large-scale outflows in star-forming galaxies are observed to be ubiquitous and are a key aspect of theoretical modeling of galactic evolution, the focus of the Simulating Multiscale Astrophysics to Understand Galaxies (SMAUG) project. Gas blown out from galactic disks, similar to gas within galaxies, consists of multiple phases with large contrasts of density, temperature, and other properties. To study multiphase outflows as emergent phenomena, we run a suite of rougly parsec-resolution local galactic disk simulations using the TIGRESS framework. Explicit modeling of the interstellar medium (ISM), including star formation and self-consistent radiative heating plus supernova feedback, regulates ISM properties and drives the outflow. We investigate the scaling of outflow mass, momentum, energy, and metal loading factors with galactic disk properties, including star formation rate (SFR) surface density (Σ_SFR ∼ 10^-4 - 1 M_⊙ kpc^-2 yr^-1), gas surface density ( ${{\rm{\Sigma }}}_{\mathrm{gas}}\sim 1\mbox{--}100\,{M}_{\odot }\,{\mathrm{pc}}^{-2}$ ), and total midplane pressure (or weight; ${P}_{\mathrm{mid}}\approx { \mathcal W }\sim {10}^{3}\mbox{--}{10}^{6}\,{k}_{B}\,{\mathrm{cm}}^{-3}\,{\rm{K}}$ ). The main components of outflowing gas are mass-delivering cool gas (T ∼ 10⁴ K) and energy/metal-delivering hot gas (T ≳ 10⁶ K). Cool mass outflow rates measured at outflow launch points (one or two scale heights $\sim 300\,\mathrm{pc}\mbox{--}1\,\mathrm{kpc}$ ) are 1-100 times the SFR (decreasing with Σ_SFR), although in massive galaxies most mass falls back owing to insufficient outflow velocity. The hot galactic outflow carries mass comparable to 10% of the SFR, together with 10%-20% of the energy and 30%-60% of the metal mass injected by SN feedback. Importantly, our analysis demonstrates that in any physically motivated cosmological wind model it is crucial to include at least two distinct thermal wind components.