Hydrodynamical Coupling of Mass and Momentum in Multiphase Galactic Winds

Abstract

Using a set of high resolution hydrodynamical simulations run with the Cholla code, we investigate how mass and momentum couple to the multiphase components of galactic winds. The simulations model the interaction between a hot wind driven by supernova explosions and a cooler, denser cloud of interstellar or circumgalactic media. By resolving scales of $\Delta x<0.1$ pc over $>100$ pc distances our calculations capture how the cloud disruption leads to a distribution of densities and temperatures in the resulting multiphase outflow, and quantify the mass and momentum associated with each phase. We find the multiphase wind contains comparable mass and momenta in phases over a wide range of densities and temperatures extending from the hot wind ($n \approx 10^{-2.5}$ $\mathrm{cm}^{-3}$, $T \approx 10^{6.5}$ K) to the coldest components ($n \approx 10^2$ $\mathrm{cm}^{-3}$, $T \approx 10^2$ K). We further find that the momentum distributes roughly in proportion to the mass in each phase, and the mass-loading of the hot phase by the destruction of cold, dense material is an efficient process. These results provide new insight into the physical origin of observed multiphase galactic outflows, and inform galaxy formation models that include coarser treatments of galactic winds. Our results confirm that cool gas observed in outflows at large distances from the galaxy ($\gtrsim1$kpc) likely does not originate through the entrainment of cold material near the central starburst.