QUOKKA-based understanding of outflows (QED) -- III. Outflow loading and phase structure as a function of galactic environment

TL;DR

This study uses high-resolution simulations to explore how galactic outflows vary with environment, revealing three wind types and highlighting the influence of supernova scale height and cooling processes on outflow properties.

Contribution

It provides a detailed analysis of outflow loading factors and phase structures as functions of galactic parameters, offering new insights into wind classifications and their dependencies.

Findings

Identifies three broad categories of galactic winds: steady hot, multiphase bursty, and cool bursty.

Shows the ratio of supernova to gas scale height as a key determinant of wind type.

Suggests metal loading factors for Type Ia supernovae may be significantly larger than for Type II.

Abstract

We present results from a suite of 3D high-resolution hydrodynamic simulations of supernova-driven outflows from galactic disc regions with a range of gas surface density, metallicity, and supernova scale height. We use this suite to quantify how outflow properties -- particularly the loading factors for mass, metallicity, and energy -- vary with these parameters. We find that the winds fall into three broad categories: steady and hot, multiphase and moderately bursty, and cool and highly bursty. The first of these is characterised by efficient metal and energy loading but weak mass loading, the second by moderate loading of mass, metals, and energy, and the third by negligible metal and energy loading but substantial mass loading. The most important factor in determining the kind of wind a galaxy will produce is the ratio of supernova to gas gas scale heights, with the latter set by a combination of supernova rate, metallicity-dependent cooling rate, and the gravitational potential. These often combine in counterintuitive ways -- for example increased cooling causes cold clouds to sink into the galactic midplane more rapidly, lowering the volume-filling factor of dense gas and making the environment more favourable for strong winds. Our findings suggest that the nature of galactic winds is likely highly sensitive to phenomena such as runaway stars occuring at a large height and dense gas and are poorly captured in most simulations, and that metal loading factors for type Ia supernovae may be substantially larger than those for type II, with important implications for galactic chemical evolution.