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 η m , and metals η 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) their energy flux is 2-20 times greater, and (ii) their specific energy e s,h is 10-1000 times higher. Moreover, the properties of hot outflows are remarkably simple: e s,h ∝ η E,h /η m,h is almost invariant over four orders of magnitude of star formation surface density. Also, we find tentatively that η E,h /η Z,h ∼ 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 halo [M ] 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.

Characterizing mass, momentum, energy, and metal outflow rates of multiphase galactic winds in the FIRE-2 cosmological simulations

“…The lack of a strong correlation with 𝑓 dense may reflect the fact that more powerful starbursts are also expected to occur when dense ISM gas fractions are higher, and these in turn may drive more powerful winds despite high 𝑓 dense . On the other hand, our overly simplistic definition of 𝑓 dense using only particles with 𝑛 > 1000 cm −3 may not be the best diagnostic of ISM breakout conditions: if the warmer volume-filling ISM phase fraction can be reliably measured, that may lead to a more robust correlation (Li & Bryan 2020). On a related note, the ISM may be more turbulent when the overall gas fraction 𝑀 gas /(𝑀 gas + 𝑀 * ) is higher, which may make it easier to drive strong outflows (this may help explain why winds become weaker in more massive halos at later times, when their overall gas fractions have decreased; Hayward & Hopkins 2017).…”

“…Finally, we take 10 Our 𝑓 dense statistic is almost certainly too simplistic to capture the full complexity of the multi-phase ISM. A more robust measure of wind breakout conditions would take into account the full temperature-density distribution of the ISM to identify the warmer volume-filling phase fraction (e.g., Li & Bryan 2020). However, it may be challenging to account for the com-the 𝑡 cool /𝑡 ff ratio at 0.1𝑅 vir from Stern et al (2020), who analyzed the same simulations.…”

“…In addition to correlating the loading factors against the global halo virial velocity and stellar mass, it is important to consider instantaneous correlations with properties that explicitly characterize the state of the ISM and inner halo (e.g., as suggested by Fielding et al 2017b;Li & Bryan 2020). After all, the virial velocity and stellar mass correlations alone do not unambiguously explain what sets the properties of winds upon initial breakout from the ISM.…”

“…The explicit calculation of energy and momentum loading factors can help test whether winds are energy-driven or momentum-driven in a simple way, and help to interpret any secondary heating or "pushing" effects on the CGM/IGM. In recent years, small-scale, high-resolution idealized simulations have made quantitative predictions for energy and momentum loadings, with a common finding that the cold phase carries most of the mass whereas the hot phase carries most of the energy (Kim & Ostriker 2018;Fielding et al 2018;Hu 2019;Li & Bryan 2020;Kim et al 2020a). These idealized numerical experiments have also been able to correlate their loading factors against the granular conditions of the ISM in which SNe go off rather than just the global halo circular velocity (e.g., Creasey et al 2013;Fielding et al 2017b;Li & Bryan 2020;Kim et al 2020a).…”

“…In recent years, small-scale, high-resolution idealized simulations have made quantitative predictions for energy and momentum loadings, with a common finding that the cold phase carries most of the mass whereas the hot phase carries most of the energy (Kim & Ostriker 2018;Fielding et al 2018;Hu 2019;Li & Bryan 2020;Kim et al 2020a). These idealized numerical experiments have also been able to correlate their loading factors against the granular conditions of the ISM in which SNe go off rather than just the global halo circular velocity (e.g., Creasey et al 2013;Fielding et al 2017b;Li & Bryan 2020;Kim et al 2020a). A similarly comprehensive analysis of multi-phase galactic winds in cosmological simulations would provide major insights on how "ejective feedback" (quantified by mass and metal loading) and "preventative feedback" (quantified by energy and momentum loading) may act in concert to regulate galaxy evolution.…”