Supernova-driven galactic winds

by Wolfgang von Glasow

Supernova-driven Galactic Winds

Galactic winds are commonly characterised by supersonic, biconically shaped outflows perpendicular to the midplane of a galactic disc, energetic enough to carry significant amounts of gas far away from their host system, and in some cases even into the intergalactic medium. We perform hydrodynamical simulations of a young galactic disc embedded in a hot gaseous halo, using the 3D grid code NIRVANA. We take into account the (static) gravitational potentials due to a dark matter halo, a stellar bulge and a disc of stars and gas. Star formation is treated by a local Kennicutt-Schmidt law. Supernovae (SNe) are triggered randomly and have preset event sizes of several tens to hundreds. We further investigate different halo gas pressures and energy injection methods.

In this movie we show the mass density distribution of one of our simulations. We assume one SN per 100 solar masses of newly formed stars to occur subsequently. The SNe are comprised into bubble-producing events of 100 SNe per event. They release an energy of 1051 erg per SN in the form of a Sedov-Taylor blast wave, where 40 per cent of the energy are released in kinetic, and 60 per cent in form of thermal energy.
We can clearly discern individual superbubbles expanding already at 10 Myr beyond a height of 1 kpc above and below the disc. These bubbles keep expanding, driven by their overpressure against the radially quickly declining halo pressure. At 40 Myr the superbubbles unite, creating a low density funnel close to the axis of symmetry. Since the gas inside this structure provides less resistance to subsequently escaping superbubbles than the rest of the halo region, material from the succeeding bubbles will continue to flow at ease through the funnel. The latter is surrounded by a conical structure of notably denser material which was originally entrained from the dense disc by outgoing shock fronts and hence continues to move outwards. Over time, enormous amounts of SN energy are fed into the disc, which in turn becomes extremely turbulent: large portions of gas are torn out of the disc midplane, partially due to entrainment by the wind, but eventually fall back. The shape of the disc gets highly irregular and clumpy but the disc remains overall intact.
The bulk velocities in the wind gas may easily reach escape velocity. Due to the geometrical constraint from the galactic gas disc, the outflow becomes conical. The escape velocity at 10 kpc distance from the disc amounts to vesc = 426 km/s, which is well below the typical wind velocities close to 103 km/s.

Many of our simulated galaxies, but not all, develop bipolar outflows. We characterise the strength of the outflow by mass and energy outflow rates, and investigate the effect of changes to the details of the model. We find that supernovae are more effective if comprised into larger superbubbles. The weight of the halo gas is able to quench galactic outflows. Buoyancy, though having a measurable effect, and clearly helpful to get superbubbles out of the disc, is too weak to drive the wind by itself. Thermal energy is found to be the dominant wind driver in our simulations. Overall, we find rather low mass and energy outflow rates which do not exceed the star formation rate and about ten percent of the energy injection rate, respectively. The latter finding potentially disagrees with observations and might thus point to a missing element in our simulations.