Simple Yet Powerful: Hot Galactic Outflows Driven by Supernovae

TL;DR

This paper synthesizes small-scale simulations to reveal that hot galactic outflows driven by supernovae are dominant in energy and follow simple scaling relations, crucial for galaxy formation models.

Contribution

It identifies universal properties of hot outflows and proposes their explicit inclusion in cosmological galaxy formation simulations.

Findings

Hot outflows dominate energy flux over cool outflows.

Specific energy of hot outflows is 10-1000 times higher than cool outflows.

A simple relation links hot outflow properties to galaxy mass and formation conditions.

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 $M_{\rm{halo}}\ [M_\odot] \gtrsim 12$. 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.