Simple Yet Powerful: Hot Galactic Outflows Driven by Supernovae

Abstract

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 , and metals . 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: is almost invariant over four orders of magnitude of star formation surface density. Also, we find tentatively that 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 indicates that hot outflows cannot escape dark matter halos with log . 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.