Stellar Feedback in Galaxies and the Origin of Galaxy-scale Winds

TL;DR

This paper demonstrates how various stellar feedback mechanisms drive galaxy-scale winds, with their relative importance depending on galaxy type and properties, and provides new fitting functions for use in larger-scale models.

Contribution

It introduces comprehensive numerical methods for stellar feedback in galaxy simulations and quantifies the contributions of different feedback processes to galactic winds.

Findings

Galactic wind outflow rates reach 10-20 times the SFR.

Radiation pressure dominates in massive, gas-rich systems.

Supernovae and stellar winds dominate in low-density dwarf galaxies.

Abstract

Feedback from massive stars is believed to play a critical role in driving galactic super-winds that enrich the IGM and shape the galaxy mass function and mass-metallicity relation. In previous papers, we introduced new numerical methods for implementing stellar feedback on sub-GMC through galactic scales in galaxy simulations. This includes radiation pressure (UV through IR), SNe (Type-I & II), stellar winds ('fast' O-star through 'slow' AGB winds), and HII photoionization. Here, we show that these feedback mechanisms drive galactic winds with outflow rates as high as ~10-20 times the galaxy SFR. The mass-loading efficiency (wind mass loss rate divided by SFR) scales inversely with circular velocity, consistent with momentum-conservation expectations. We study the contributions of each feedback mechanism to galactic winds in a range of galaxy models, from SMC-like dwarfs & MW-analogues to z~2 clumpy disks. In massive, gas-rich systems (local starbursts and high-z galaxies), radiation pressure dominates the wind generation. For MW-like spirals and dwarf galaxies the gas densities are much lower, and shock-heated gas from SNe and stellar winds dominates production of large-scale outflows. In all models, however, winds have a multi-phase structure that depends on interactions between multiple feedback mechanisms operating on different spatial & time scales: any single mechanism fails to reproduce the winds observed. We provide fitting functions for wind mass-loading and velocities as a function of galaxy properties, for use in cosmological simulations and semi-analytic models. These differ from typically-adopted formulae with explicit dependence on gas surface density that can be very important in both low-density dwarf galaxies and high-density gas-rich galaxies.