QED VI: Neutral hydrogen gas in supernova-driven galactic outflows

Winds that eject mass from galactic discs regulate the growth of galaxies, and observations show that, at least within a few kpc of the disc, most of the mass flux is carried by neutral rather than ionized material. However, it has proven difficult for simulations of supernova-driven winds to reproduce this observation, leading to speculations that the bulk of the mass must be driven by radiation pressure or cosmic rays rather than supernovae. Here we report simulations of supernova-driven winds using the GPU-accelerated code QUOKKA that reach a resolution of $Δx = 1$ pc, significantly higher resolution and simulation volume than previous work. At this resolution the winds are dominated by neutral mass, with cloud sizes, masses and column densities broadly similar to those of the HI clouds in the Milky Way's nuclear wind. The neutral material is not ballistic, but is instead driven by momentum transfer associated with drag and condensation of hot gas onto the neutral phase. By contrast, when we coarsen the resolution to values comparable to those used in earlier work, the neutral outflow weakens at $Δx = 2$ pc and disappears at $Δx = 4$ pc. We argue that this difference arises partly because lower-resolution simulations do not resolve, with a few cells, the 3-6 pc thick cooling layers in which hot gas condenses onto neutral clouds.