Simple Yet Powerful: Hot Galactic Outflows Driven by Supernovae

Supernovae (SNe) drive multiphase galactic outflows, impacting galaxy formation; however, cosmological simulations mostly use ad hoc feedback models for outflows, making outflow-related predictions from first principles problematic. Recent small-box simulations resolve individual SNe remnants in the interstellar medium (ISM), naturally driving outflows and permitting a determination of the wind loading factors of energy η_E, mass ${\eta }_{m}$ , and metals ${\eta }_{Z}$ . In this Letter, we compile small-box results, and find consensus that the hot outflows are much more powerful than the cool outflows: (I) hot outflows generally dominate the energy flux, and (II) their specific energy e_s,h is 10-1000 times higher than cool outflows. Moreover, the properties of hot outflows are remarkably simple: ${e}_{s,h}\propto {\eta }_{E,h}/{\eta }_{m,h}$ is almost invariant over four orders of magnitude of star formation surface density. Also, we find tentatively that ${\eta }_{E,h}/{\eta }_{Z,h}\sim $ 0.5. If corroborated by more simulation data, these correlations reduce the three hot phase loading factors into one. Finally, this one parameter is closely related to whether the ISM has a "breakout" condition. The narrow range of ${e}_{s,h}$ indicates that hot outflows cannot escape dark matter halos with log ${M}_{\mathrm{halo}}\ [{M}_{\odot }]\gtrsim 12$ . This mass is also where the galaxy mass-metallicity relation reaches its plateau, implying a deep connection between hot outflows and galaxy formation. We argue that hot outflows should be included explicitly in cosmological simulations and (semi-)analytic modeling of galaxy formation.