Resolving The Generation of Starburst Winds in Galaxy Mergers

TL;DR

This study uses high-resolution simulations to analyze galaxy merger-driven winds, revealing their complex structures, dependence on galaxy properties, and impact on star formation, with implications for galaxy evolution models.

Contribution

It provides detailed, self-consistent models of stellar feedback in galaxy mergers, resolving limitations of sub-grid models and exploring wind morphology, velocity, and effects on star formation.

Findings

Winds are multi-phase, complex, and depend on galaxy properties.

Wind mass-loading efficiency is similar in mergers and isolated galaxies.

Post-merger star formation can be sustained by fallback of wind material.

Abstract

We study galaxy super-winds driven in major mergers, using pc-resolution simulations with detailed models for stellar feedback that can self-consistently follow the formation/destruction of GMCs and generation of winds. The models include molecular cooling, star formation at high densities in GMCs, and gas recycling and feedback from SNe (I&II), stellar winds, and radiation pressure. We study mergers of systems from SMC-like dwarfs and Milky Way analogues to z~2 starburst disks. Multi-phase super-winds are generated in all passages, with outflow rates up to ~1000 M_sun/yr. However, the wind mass-loading efficiency (outflow rate divided by SFR) is similar to that in isolated galaxy counterparts of each merger: it depends more on global galaxy properties (mass, size, escape velocity) than on the dynamical state of the merger. Winds tend to be bi- or uni-polar, but multiple 'events' build up complex morphologies with overlapping, differently-oriented bubbles/shells at a range of radii. The winds have complex velocity and phase structure, with material at a range of speeds up to ~1000 km/s, and a mix of molecular, ionized, and hot gas that depends on galaxy properties and different feedback mechanisms. These simulations resolve a problem in some 'sub-grid' models, where simple wind prescriptions can dramatically suppress merger-induced starbursts. But despite large mass-loading factors (>~10) in the winds, the peak SFRs are comparable to those in 'no wind' simulations. Wind acceleration does not act equally, so cold dense gas can still lose angular momentum and form stars, while blowing out gas that would not have participated in the starburst in the first place. Considerable wind material is not unbound, and falls back on the disk at later times post-merger, leading to higher post-starburst SFRs in the presence of stellar feedback. This may require AGN feedback to explain galaxy quenching.