Simple Yet Powerful: Hot Galactic Outflows Driven by Supernovae

Abstract

Supernovae (SNe) drive multiphase galactic outflows, impacting galaxy formation; however, cosmological simulations mostly use \textit{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 ${\eta }_E$, 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 \esh\ 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 \textit{hot} outflows and galaxy formation. We argue that hot outflows should be included explicitly in cosmological simulations and (semi-)analytic modeling of galaxy formation.

9 pages, 4 figures, accepted by ApJL. Compilation of loading factors of supernovae-driven outflows is at https://github.com/limiao0611/loading-factors/wiki

References in corpus (9)

• The EAGLE project: Simulating the evolution and assembly of galaxies and their environments
• Theory of Star Formation
• The Circumgalactic Medium
• The Mass-Metallicity Relation at z~2
• Momentum Injection by Supernovae in the Interstellar Medium
• Moving mesh cosmology: tracing cosmological gas accretion
• Superbubbles in the Multiphase ISM and the Loading of Galactic Winds
• The metallicity of galactic winds
• Launching of hot gas outflow by disc-wide supernova explosions

Cited by in corpus (1)

• First results from SMAUG: Characterization of Multiphase Galactic Outflows from a Suite of Local Star-Forming Galactic Disk Simulations